Update Base URL to only accept the correct URL

[husjon]

The Base URL input should automatically capture the required parameters.

The accepted URL as mentioned in the readme is https://www.goodreads.com/review/list_rss/<USER_ID>?key=<MY_KEY>&shelf=, whereas everything after &shelf= should be omitted as it will be handled by the Your Goodread shelfs option.

I think doing a regex capture of the above URL and stripping off anything else would help reducing the chance of mistakes (e.g #14).