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.
The userid portion of the URL should probably be made optional – it's always given when clicking the `share` button on SE, but sometimes other sites will strip away this to protect their privacy.