Add parameter dynamic_width for music block

[UnkwUsr]

By default, the music block has a static width (= max_width), and when the name of the music is too short, it looks very ugly: just unnecessary free space inside the bar.

When dynamic_width is set to true and the name of the music is longer than max_width, block width will be set to max_width (marquee option remains available), but when name is less than max_width, block width will be set equal to the name of the music, so no free space left.
P.S. I have the music block in the first place (counting from the left), so dynamic width looks very cool. But if people have it in the middle or at the end, then music block will affect next blocks after music and jumpy text can be annoying, so we have this behavior as a config parameter.

Possible resolves #780

[Alexeyan]

+1 from me. Would like this feature.

[ammgws]

Seems OK to me.