TODO list:
 * cache the config group entry, don't read it for every parse job
 * do PEP8 checking for open documents only, or when the document gets
   activated! Probably we want to attach a flag to the top context or so.
 * refresh checking on all whitespace changes
It's possibly quite slow... needs more testing.
The feature is disabled by default.

Conflicts:
	CMakeLists.txt