improve naming consistency

users.py/skills.py: various method names changed to use snake_case
skills.py: add more detail to progress printout
replace menu-based interface

Ripped out the menu interface in favor of a traditional CLI. I
was originally going to keep both as the menu interface can be rather
convenient; however, I decided I didn't want to maintain both.

The new interface is influenced by todo.txt's CLI, where to modify any
todo you enter its number.