Added structure for common elements to workouts
Changed getWorkout to read in common features of workouts

Used to have just difficulty as the common feauture. This commit
adds support to read in a Common type from stdin, allowing us to
also read in a Workout with Common fields and WorkoutType fields.

This was pretty easy to do and I like the way I structure this.
Added support for showing options for some fields

In fields that have a fixed set of inputs (such as WortoutType),
entering ... will now show you the available inputs
To avoid accidentally entering this in other fields that take Strings,
String fields must now take at least 1 alpha-numeric character