Support expansion of `%USERPROFILE%` and document paths taking `/` on Windows

[whisperity]

Using the Windows-specific \ path separator in the config YAML results in error:

[error] Configuration file is corrupted. yaml-cpp: error at line 1211, column 33: unknown escape character: P

Using / instead solves this issue. However, the default help text or value only shows a Mac-specific example: /Users/trapni/Pictures/bg.png.

It would be nice if the expansion of %USERPROFILE% (to C:/Users/Whisperity) would be supported. Currently, using %USERPROFILE% in the configuration for the path results in [error] Background image not found: %USERPROFILE%/Pictures/....

Alternatively, it would be even better if the expansion of ~ to mean %USERPFORILE% and expand to the same location (i.e., C:\Users\Whisperity), just like Git Bash does it:

whisperity@WVM MINGW64 ~
$ pwd
/c/Users/Whisperity

would be even better, as, assuming the user sets up the same directory structure for resources on both platforms, could re-use the same configuration file cross-platform. I would definitely like not having to edit the config file on Windows just to fix the background image's path and then constantly see as if I have uncommitted changes to my, otherwise versioned, configuration.

[christianparpart]

Since ~ home directory expansion is already working, we decided on Discord, to implement environment variable substitution after 0.4.0 release.

[christianparpart]

What do you mean by "and document paths taking / on Windows" btw?

[christianparpart]

@whisperity what syntax of variable expansion do you have in mind? I'd try not to use %EnvName% syntax, because that's waaay to outdated. It's also easier to parse ${EnvName} instead, and users are probably used to see that syntax already anyways, regardless of the underlying operating system.
What do you think?