Use parameterized colors in ANSIDocs for headings

Instead of calling IEx.Options.get(:colors) directly, set it as default value for the colors parameter.
This way, when the rest of the module start using the parameterized colors, it can be decoupled from IEx.Options (and IEx.color).
Pass colors parameter to process* functions in ANSIDocs
Use parameterized colors in ANSIDocs for body headings
Use parameterized colors in ANSIDocs for code
Pass colors parameter to inline handling functions in ANSIDocs
Use parameterized colors in ANSIDocs for inline handling
Refactor ANSIDocs.write helper