Move all parser definitions to powerline.commands.*.get_argparser()

Reasoning: they will be easier to reach there. It will also be possible to use
specific ArgumentParser class that will just collect data for sphinx.
Improve powerline-config help and require --shell argument
Make all metavars uppercased
Add `man` documentation target

Fixes #1131
Add new documents to the main TOC tree
Also add documentation for `powerline-lint`