Added force_quotes property to MarkupCommand.

Set to true to force quotes around all markup command argument strings instead
of around just markup command argument strings that contain a space.

Use to make markup with consistently quoted arguments for very clean .ly files.

Examine the treatment of 'EvenRunRhythmMaker(' in the two examples below.

With forced_quotes=False (default):

    ^ \markup {
        \override
            #'(font-name . "Courier")
            \column
                {
                    EvenRunRhythmMaker(
                    "    beam_specifier=BeamSpecifier("
                    "        beam_each_division=True,"
                    "        beam_divisions_together=True,"
                    "        ),"
                    "    )"
                }
        }

With forced_quotes=True (new option):

    ^ \markup {
        \override
            #'(font-name . "Courier")
            \column
                {
                    "EvenRunRhythmMaker("
                    "    beam_specifier=BeamSpecifier("
                    "        beam_each_division=True,"
                    "        beam_divisions_together=True,"
                    "        ),"
                    "    )"
                }
        }