Aliases for commands

[ThePaSch]

This implements the ability to define aliases for any command. Command objects now take an additional member "aliases", an array of strings, which can be used in place of the command's full name and will function just the same.

Cleanup and resubmission of #14.

[coveralls]

Changes Unknown when pulling c5364ade04966239bb1236d0e63850278bd87147 on ThePaSch:master into ** on MeLlamoPablo:master**.

[MeLlamoPablo]

Thanks for the PR!

The only thing left is adding tests to cover the new feature and maybe try to get coverage back to 100%. If you don't want to do that; it's ok. I'll do it whenever I can, and merge the PR.

Otherwise, let me know if you're going to do it so we don't waste any effort. Here you can see the uncovered lines.