Refactored Markup.with_color() and Markup.smaller() as methods.

OLD:

    markuptools.with_color()
    markuptools.smaller()

NEW:

    Markup.with_color()
    Markup.smaller()

Chaining like this ...

    markup = Markup('complicated text')
    markup = markup.with_color('blue')
    markup = markup.smaller()

... is now awesome.

MODEL: Markup chaining harmonizes with selector chaining.