docs: add docs for lunacolors (closes #144)

docs/lunacolors.txt

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+Lunacolors is an ANSI color/styling library for Lua. It is included
+by default in standard Hilbish distributions to provide easy styling
+for things like prompts and text.
+
+For simple usage, a single color or style is enough. For example,
+you can just use `lunacolors.blue 'Hello world'` and that'll return
+blue text which you can print. This includes styles like bold,
+underline, etc.
+
+In other usage, you may want to use a format string instead of having
+multiple nested functions for different styles. This is where the format
+function comes in. You can used named keywords to style a section of text.
+
+The list of arguments are:
+Colors:
+- black
+- red
+- green
+- yellow
+- blue
+- magenta
+- cyan
+- white
+Styles:
+- bold
+- dim
+- italic
+- underline
+- invert
+
+For the colors, there are background and bright variants. The background
+color variants have a suffix of `Bg` and bright has a prefix of `bright`.
+Note that appropriate camel casing has to be applied to them. So bright
+blue would be `brightBlue` and background cyan would be `cyanBg`.