feat: new option to show Glow in same window

The option `in_place` determines if the output from Glow has to be
displayed in the same window as the input buffer or instead of
in a preview window like it was before this commit.

The ability to manage multiple windows with different markdown buffers
was incidentally implemented.

The `in_place_state` table is a map where:
- a key represent a certain window
- its value contains both the input buffer and the output buffer (the
  one created by *glow.nvim* holding the content of Glow).

```lua
in_place_state = {
    [WIN_1] = { BUFFER_BEFORE, BUFFER_CREATED },
    ...
}
```

So to the steps now are:
- Check if the config flag is enabled
- If it is save the state to retrieve BUFFER_BEFORE when needed
- Replace the buffer in WIN_1 with BUFFER_CREATED
- When "closing the window" reset the BUFFER_BEFORE in the WIN_1 and set
  in_place_state[WIN_1] to NIL

Line 346 had to be changed as well or:
- `Glow|Glow!|Glow` wouldn't have been possible (e.g. use command
  multiple times)
- `Glow|Glow` printed an error:

```lua
(current_win == win and not glow.config.in_place) -- keep previous
behaviour
or
in_place_state[current_win] -- Window is managed by Glow and needs to be
considered as a `glowpreview` buffer (e.g. closed or not bothered)
```

This is because before `current_win == win` always meant we were on the
preview window which now isn't the case anymore.

Author: lnk3

README.md

@@ -9,7 +9,7 @@
   </p>
   <img src="https://img.shields.io/badge/Made%20with%20Lua-blueviolet.svg?style=for-the-badge&logo=lua" />
   <img src="https://img.shields.io/github/actions/workflow/status/ellisonleao/glow.nvim/default.yml?style=for-the-badge" />
-  
+
 </div>
 
 https://user-images.githubusercontent.com/178641/215353259-eb8688fb-5600-4b95-89a2-0f286e3b6441.mp4
@@ -60,6 +60,7 @@ The script comes with the following defaults:
   height = 100,
   width_ratio = 0.7, -- maximum width of the Glow window compared to the nvim window size (overrides `width`)
   height_ratio = 0.7,
+  in_place = false,
 }
 ```
 
@@ -94,10 +95,10 @@ require('glow').setup({
 :Glow
 ```
 
-### Close window
+### Close window or return to input buffer
 
 ```
 :Glow!
 ```
 
-You can also close the floating window using `q` or `<Esc>` keys
+You can also close the floating window or go back to the initial buffer using `q` or `<Esc>` keys

lua/glow.lua

@@ -1,4 +1,5 @@
 local win, buf, tmpfile
+local in_place_state = {}
 local job = {}
 local glow = {}
 
@@ -12,6 +13,7 @@ glow.config = {
   pager = false,
   width = 100,
   height = 100,
+  in_place = false,
 }
 
 local function cleanup()
@@ -51,7 +53,16 @@ end
 local function close_window()
   stop_job()
   cleanup()
-  vim.api.nvim_win_close(win, true)
+  if not glow.config.in_place then
+    vim.api.nvim_win_close(win, true)
+  else
+    local to_close_win = vim.fn.win_getid()
+    local managed = in_place_state[to_close_win]
+    if managed then
+        vim.api.nvim_win_set_buf(to_close_win, managed[1])
+        in_place_state[to_close_win] = nil
+    end
+  end
 end
 
 local function tmp_file()
@@ -99,10 +110,20 @@ local function open_window(cmd_args)
 
   -- create preview buffer and set local options
   buf = vim.api.nvim_create_buf(false, true)
-  win = vim.api.nvim_open_win(buf, true, win_opts)
+
+  if not glow.config.in_place then
+    win = vim.api.nvim_open_win(buf, true, win_opts)
+    vim.api.nvim_win_set_option(win, "winblend", 0)
+  else
+    local win_in_place = vim.api.nvim_get_current_win()
+    local buf_in_place = vim.api.nvim_get_current_buf()
+
+    in_place_state[win_in_place] = { buf_in_place, buf }
+
+    vim.api.nvim_win_set_buf(win_in_place, buf)
+  end
 
   -- options
-  vim.api.nvim_win_set_option(win, "winblend", 0)
   vim.api.nvim_buf_set_option(buf, "bufhidden", "wipe")
   vim.api.nvim_buf_set_option(buf, "filetype", "glowpreview")
 
@@ -343,7 +364,7 @@ glow.execute = function(opts)
   end
 
   local current_win = vim.fn.win_getid()
-  if current_win == win then
+  if ( current_win == win and not glow.config.in_place) or in_place_state[current_win] then
     if opts.bang then
       close_window()
     end

tests/glow/glow_spec.lua

@@ -12,6 +12,7 @@ describe("setup", function()
       pager = false,
       width = 100,
       height = 100,
+      in_place = false,
     }
     glow.setup()
     assert.are.same(glow.config, expected)
@@ -27,6 +28,7 @@ describe("setup", function()
       mouse = false,
       width = 200,
       height = 100,
+      in_place = true,
     }
     glow.setup(expected)
     assert.are.same(glow.config, expected)