Custom JS scripts for UnTab actions

[blenderskool]

Explore ways in which UnTab can execute custom user scripts in the context of a browser tab.
This could be something similar to bookmarklets(which was discussed quite a bit in #60) or normal JS snippets that is injected into a tab to perform some action.

The user should be aware of the kind of script they are running as the scripts may have complete control over the webpage it is executed on.