Visit question from link

This implements a few functions. The most important one is the `sx-open-link` command. 

Now we can call this command, paste in a link, and we will visit this link inside `sx`.

Should really help with checking on issues that happen on a specific question.
It matches the end-of-string. It's somewhat similar to `$`. Except `$` will also match before a `\n`, and `\\'` **only** matches at the end of the string.