Add option for displaying completion information

This patch adds support for a --help-completion option, which will
display the defined options and their completion information, in a
format designed to be parsed easily from Python, for integration into
build-bash-completion.

The alternative would have been to duplicate the build-bash-completion
functionality on the Haskell side, but that would have been lots of
duplication for not enough gain (since that is only run at build
time).

Signed-off-by: Iustin Pop <iustin@google.com>
Reviewed-by: Agata Murawska <agatamurawska@google.com>