Introduce generic multi-command binary handling

Currently, the mon-collector binary is the only multi-command binary
we have in Haskell. Not surprisingly therefore, its command line
handling is not as robust as we need, not being able to support
standard "--help" commands.

This patch introduces generic multi-command handling, similar to the
Python functionality (some parts missing, will be added in future
patches), and switches mon-collector over to it.

Signed-off-by: Iustin Pop <iustin@google.com>
Reviewed-by: Michele Tartara <mtartara@google.com>