Fixes for base16 theme

[skipkayhil]

Background

Commits are split into refactor/changes/additions, so reviewing them individually should be much easier than the final diff.

Base16 ruby code highlighting reference: https://base16.vercel.app/previews/base16-default-dark

There are still a few discrepancies between the After and the reference highlighting:

• In the base16 reference, require is highlighted like a keyword and attr_accessor, print, and puts are highlighted like methods, but the Rouge ruby lexer defines all of these as Name::Builtin. I think most of the Rouge's ruby Name::Builtin should just be removed so that they can just be methods, but that isn't related to the Base16 theme.
• self is correctly highlighted like a keyword in the reference, but the lexer currently says its a Name::Class.
• Rouge has Person::name as a Name, but I think it should be possible to update the lexer to correctly identity them as Name::Function

Example Screenshots

Before:

After:

[skipkayhil]

hey @tancnle, anything I can do to help get this merged?