Input for mibbit-key on settings page

Issue Summary

Would be cool to be able to input your own mibbit widget key on the settings-page that you get after registering on their site and creating your own widget. The advantage is that you can set a couple of preferences and colours, so the widget better integrates with the rest of the site...

It wouldn't be too hard to implement, but I've currently just hardcoded it on my site...
What we'll have to do is to add an input-box that saves the key on the settings page. The next step is to add a if-statement that checks if you have entered a key or not and depending on that either outputs your personal widget or the regular widget as used now... :)