feat(expressions): uses oniguruma regexp for colors search

Buy using Oniguruma instead of native RegExp, regexp searching for
color names can now use negative lookbehind.

BREAKING CHANGE: custom color expression can no longer rely on RegExp.

    To migrate the code follow the example below:

    Before:

    Color.addExpression /#([\da-fA-F]{6})(?![\da-fA-F])/, (color,
expression) ->
      [m, hexa] = @regexp.exec(expression)

      color.hex = hexa

    After:

    Color.addExpression "#(#{hexa}{6})(?!#{hexa})", (color, expression)
->
      [m, hexa] = @onigRegExp.search(expression)

      color.hex = hexa.match
style(markers): uses inline-block marker instead of block with width
docs(README): updates documentation with oniguruma examples