Some documentation and lint checker improvements

Fixes to lint checker:

- `vim` and `ipython` extensions are now optional.
- `highlight_group` or `name` is required to be present in non-function 
  segments; previously only `highlight_group` was required
- Extended checking for `args` dict: it now checks whether all required keys are 
  present and checks whether other present keys can be accepted.

Documentation fixes:

- Removed `pl`, `update_value` (and renamings) arguments from documentation.
- Fixed documentation for `ThreadedSegment` subclasses with static methods (it 
  was not removing some arguments).
- Added more renderer docstrings.

Configuration changes:

- Made `None` priority special in place of any priority that is less or equal 
  then zero.
- Made it look up `module.name` in addition to `name` in colorschemes.

Other:
- Renamed `PowerlineState` to `PowerlineLogger`.