Fix short and long help text display logic

While working on other features, I removed the last help command in the
interpreter. This broke a unit test and uncovered a bit of broken logic.
Now the short and long help text display correctly under the more robust
conditions.

Conditions:
* when `shazam` is called, show the short text
* when `shazam` is called on empty text, show the short text
* when `shazam help` is called, show the long text
Begin writing add and subtract routines
Add tests for keywords