ANSI 256-color and 24-bit color support

[dahlbyk]

This PR adds support for colors beyond [ConsoleColor]:

• [byte] color values are processed as as Xterm 256 colors
• [System.Drawing.Color] color values are processed as 24-bit colors
• [string] values are parsed as HTML color values and processed as 24-bit colors

Includes #454 for adjusting Write-Prompt semantics, updated for 1.0 changes.

24-bit support currently depends on System.Drawing. Not sure what Core options are there, but I've tried to make the assembly/type loading resilient.

Usage

$GitPromptSettings.DefaultForegroundColor = [ConsoleColor]::Blue
$GitPromptSettings.DefaultForegroundColor = 'Aquamarine'
$GitPromptSettings.DefaultForegroundColor = [Drawing.Color]::Aqua
$GitPromptSettings.DefaultForegroundColor = '#bbb'
$GitPromptSettings.DefaultForegroundColor = [byte]0xf0
$GitPromptSettings.DefaultForegroundColor = $null

I have tried testing with ConEmu, but 256-color mode doesn't seem to be working? Or is that a limitation of PS v5?

[dahlbyk]

Interesting... so a question maybe for @lzybkr: is it expected that the $Host.UI.RawUI color properties are mutated by "<ANSI color string>" | Out-Host but not "<ANSI color string>" | Write-Host"? Since #447, using ANSI in the prompt means $Host.UI.RawUI colors can't be adjusted because they're always reset by ESC[0m in the prompt result. Are RawUI.ForegroundColor/BackgroundColor not meant to be manipulated? How else might one change, let's say, the default colors from the (very peculiar) defaults of DarkYellow/DarkMagenta?

[lzybkr]

256 color/24 bit color support is in the Creators update - hopefully hitting RTM soon. You can try out insiders builds to see it working - there's nothing special about it in PowerShell, see https://twitter.com/lzybkr/status/779069905136037888

[lzybkr]

This should be

$ColorTranslatorType = 'System.Drawing.ColorTranslator' -as [Type]

[lzybkr]

Regarding ESC[0m - I feel like there is a bug in conhost in not honoring the current foreground/background colors - but it depends on how you set those colors.

If you set fore/background colors via escape sequences, then I believe ESC[0m is working as expected - not necessarily the way I think it should, but it's compatible with *nix systems.

If you set fore/background colors via a specific Win32 api, I think ESC[0m should reset to those colors, not the initial colors from startup.

Ideally ANSI would have push/pop semantics, but that doesn't exist unfortunately.

[dahlbyk]

@lzybkr so from posh-git's standpoint, then, would you agree that it is most correct for us to continue using ESC[0m to terminate our colorized strings?

I guess what confuses me most is the distinction between Write-Host (doesn't modify RawUI) and Out-Host (does). Apparently the prompt string is using the latter?

[dahlbyk]

Merged so I can more easily bring master's Write-Prompt changes into develop.