Extend IEx.ANSIDocs.write to accept string value for style

The write/2 helper now accepts both an atom referencing a style in the IEx.Options.get(:colors) list, and a style string that can be passed straight to IO.ANSI.escape_fragment/2.

This is a transitional change. Once IEx.ANSIDocs is fully decoupled from IEx.Options (and color options are passed via parameter), the write/2 helper will only accept the string value.
Use parameterized colors in IEx.ANSIDocs.print_heading