The rbt alias command is a new command for manging your aliases

defined in .reviewboardrc files. It currently has the capability to

list all defined aliases, via rbt alias --list, and to do a "dry run"

of a command, via rbt alias --dry-run <ALIAS>.
The alias listing will show what files aliases are defined in, as well

as if they are overridden by other aliases or rbt commands (defined

by entry points or the presence of an rbt-command-name executable).
Some refactoring has been done to the rbtools.comands module to

handle command detection so that it can be reused by the new command.

Refactoring has also been performed on the rbtools.utils.aliases

module to allow aliases to be expanded without being ran.

Ran unit tests.
Tested the rbt alias --list and rbt alias --dry-run commands.

They work as expected.