bash-completion: add support for multi-cmd Haskell binaries

This patch adds support for parsing the command list out of a binary
(very strict format), and then iterating over that and building the
sub-commands options/arguments.

It also does a bit of general cleanup in the script.

Signed-off-by: Iustin Pop <iustin@google.com>
Reviewed-by: Michael Hanselmann <hansmi@google.com>