Improve help support for custom commands and aliases.
Improve help support for custom commands and aliases.

RBTools supports adding custom commands by either finding a
corresponding executable on $PATH beginning with 'rbt-', or by setting
an alias. However, these custom commands do not appear when running 'rbt
help'. Also, getting command-specific help does not work - neither 'rbt
help new-command' nor 'rbt new-command --help'.

This change fixes both of these shortcomings.

Testing Done:
Tested locally on my own ReviewBoard instance with both commands on
$PATH and aliases.  I saw both listed in `rbt help`.  I also tested `rbt
help <cmd>` and `rbt <cmd> --help` for both commands on $PATH and
aliases, and usage (or the alias string) was printed as expected.

Reviewed at https://reviews.reviewboard.org/r/11552/

reviewboard/rbtools,reviewboard/rbtools,reviewboard/rbtools