Implemented support for command-line options.

--shell [zsh|bash]
	which shell to emit color escapes for; defaults to zsh
--cwd-only
	use only the current working directory in the prompt; defaults to false
--mode [patched|compatible]
	which font mode to expect; defaults to patched

Any further arguments are presumed to be $? aka the error returned by the
previous command.
Add a --depth command-line option to control how many directory segments are shown.

And improve the truncation a bit.