Added g:powerline_config_path vim option
Updated documentation regarding overrides
Renamed colorscheme variable to colorscheme_config
Replaced 'NONE' with None for guifg/guibg.

No need in having string keys if we are in any case checking this for equality 
later instead of embedding them as-is like cterm*
Splitted colorschemes, added better gradient support

- Splitted colorschemes into colors definitions file (TODO: remove non-colors 
  definitions like weather_condition_hot and base00) and actual colorscheme.
- Removed dictionary containing groups definitions for all groups for all modes, 
  now colorscheme is queried for this each time. FIXME: introduced some problems 
  with mode coloring.
- Moved determination of colors from theme to renderer.
- Added gradients definitions (actually, only one) to new colors file.
- Made line_percent with gradient=True use new gradients.