persist sgstore.ignore to localstorage

[tooolbox]

As covered in #11

Works by adding an lt_localstorage setting, which is the name of the localstorage key that the ignore settings will be persisted to, just in case you want to have different ones in the same browser, i.e. by username or some such.

Note that rules injected via the settings will be persisted in localstorage, so changing those settings will be difficult. But this is only a problem if you are doing both (hardcoded rules plus localstorage).

I'm going to test this out before I take it off Draft.

[tooolbox]

Alright, working fine in Chrome 55.