Implemented Markup.__add__(). NEW FUNCTIONALITY.

Example 1: markup + markup:

    >>> Markup('foo') + Markup('bar')
    Markup(contents=('foo', 'bar'))

Example 2:

    >>> Markup('foo') + Markup.hspace(0.75)
    Markup(contents=('foo', MarkupCommand('hspace', 0.75)))

NEW: should be included in the 2.16 release notes.

Should become the default way that composers combine markup and markup
commands.