Handle improperly formatted URLs

** What I expected **
To be able to type `@WordPressDotComBot follow newestdomainsiteintheworld.com` and follow the blog

** What actually happened **
Nothing, not even an error. Instead I need to include the `http[s]://` on the front of the URL.

The bot should be able to more accurately detect a URL, and also provide an error response when a non-URL argument is given

Start by locating the follow command's URL parsing and argument validation, then reproduce the bare-domain example and a non-URL argument. Done means a valid domain such as newestdomainsiteintheworld.com is accepted without http[s]:// and an invalid argument receives an error response.

Written by the indexing model from the issue text.