Add tests for core functionality

[reibitto]

There are no tests right now and that makes it really easy to break things. "Add tests" is too broad of an issue, so for now the focus should probably be the Command.search functionality. The function doesn't require a broad set of dependencies, so it should be pretty easy to set up tests for. Just create a Vector of arbitrary commands and aliases and then pass in a search string and make sure the results are correct.

[reibitto]

Initial work was merged with #51. The next step is writing tests for all the other commands.