Palette support, "left" separators added, refactor code to not pass 'powerline' instance over and over 

- attempt to add palette support - colors can be selected by names in palette or by integers (like now)
- left separators added - now you can try to build also right side of powerline (missing feature is to move some segments to right or add something like 'fill space' segment)
- some parts of code was refactored to not pass 'powerline' instance over and over. Now powerline instance is passed only to 'draw' methods. To this time you can just add segments or list of segments which you want to render (functions like add_* now should return segments not do strange poewrline passing)