Replace on_*_change with callbacks generator
Move get_config_paths out from Powerline class
Rename find_config_file to _find_config_file
Add generate_config_finder function
Move _load_config body to a separate function
Rename create_renderer_kwargs to cr_kwargs for consistency
Move some code from Powerline.create_renderer to create_logger
Move code that sets config.json/common defaults to a separate function
Use powerline.shell.run_cmd to get tmux version

Closes #912
Merge pull request #917 from ZyX-I/fix-powerline-config-shell

Refactor powerline to create loggers and use run_cmd in powerline-config