[feature request] add option to use regular expressions to auto assign label

[SleepyPrince]

Similar to custom URL, it would be nice to have an option to auto assign the label according to specific urls/domains.
The format can be something like: regexp||label (or anything that can be easy to processed without confusing the user)

I looked into the code and this can be done in webRequest.onBeforeRequest before addTorrent is called.
One limitation is that originally I was planning to match the regular expressions with the origin url (instead of the torrent url). However when the extension accept the magnet address via https://torrent-control.invalid/, the origin url info is lost.
So the regular expression can only be matched to the magnet address, though not ideal but may still be useful.

I have a working local copy but I don't think my amateur code is ready for pull request.

[robertheadley]

#76