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.