Split huge powerline.lint module into multiple files
Remove context_key, init_context and context_has_marks functions
Replace new_context_item() with Context.enter_key()
Add Context.enter_item function for processing marked iterable items
Forbid most operations for Context instances
Remove unused new_context_item function
Move list_sep to powerline.lint.checks

It is the only place where it is needed.
Document powerline.lint.spec.Spec object
Move auxilary functions before specifications
Document powerline.lint.check function