rbtools aliases, which are defined in the .reviewboardrc file,

come in two forms: aliases for other rbtools commands and aliases

for system commands. The former allows for convenient short-hand for

rbt commands and the latter allows for more versatile and flexible

commands that utilize the shell. Aliases allow for positional variable

substition. rbtools aliases are only attempted to be executed after

attempting to find an internal command and external command (named

rbt-<commandname>) fail.
Add documentation for rbtools aliases. Add a link to the

documentation index to reference the documentation for rbtools

aliases.
Add unit tests to test the parameter substitution for rbtools

aliases.

Unit tests pass.