Add an optional 'plain' mode to the cwd segment.

Rationale:
A plain path like "~/projects/powerline-shell" can be easily copied&pasted, and also takes up less space in the prompt.

The new command line parameter --cwd-mode controls the display of the cwd segment:
--cwd-mode=fancy is the same as the current default
--cwd-mode=plain is the new mode to display a plain path
--cwd-mode=dironly is the same as the old parameter --cwd-only (which is still supported to keep backward compat)
Fix: include leading / in cwd when in plain mode
Merge remote-tracking branch https://github.com/milkbikis/powerline-shell/pull/156