Add ability to override configuration options

TODO: support for non-string scalars in vim overrides, ipython overrides, move
every non-ASCII symbol from every theme to the area (segment_data key) where it
can be overridden, tests.

Fixes #231
Replaced getfsize with os.stat
Fixed various flake8 errors
Moved all non-ASCII symbols into `segment_data`
Added --config_path, replaced nargs='*' with better action='append'
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.
- 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.