Adding todos with reserved words

First thing I tried to do (no pun intended) was add a todo item that began with "check..."

I realized that the command api was accepting (.+) and so I decided to make the regex slightly stricter with validating for digits. I didn't want to overcomplicate the app, but I think it is worth allowing flexibility with what I'm calling reserved words (check, rm, undo...).

PS this is my first pull request. I had a branch but I've already merged all my changes to master and deleted the branch.