Refactor powerline.segments.common.players to use multiple functions

Also adds documentation. Still have to update top-level themes.

Fixes #445
Fix mpd support
Rename highlight groups
Add `"player"` segment data
Prioritize now_playing highlight group, but remove it from colorscheme

Reasoning for first: backwards compatibility. If user defined `now_playing`
highlight group it is what should be used.

Reasoning for second: it is useless there. If user has defined its own group its
effect will not be affected, if he has not then there is nothing to talk about.
Add some workarounds for powerline-lint
Remove self.STATE_SYMBOLS reference: there is no such attribute
Merge branch 'enhance-now_playing' into develop