Add support for '$WHOAMI' in Config

[bobbystacksmash]

When loading a config, adding support for expanding a limited set of variables could be very useful. For example, when creating folders, it'll be annoying to add a user called "fred", then remember to update "C:\Users\Fred..." everywhere. Instead, setting general.whoami = "fred" should allow the use of the variable "$WHOAMI" to appear in path expressions, and have it expanded to "fred" before creating files and folders.