Allow access to local files

If you need the extension to work on local files (files that are not online) you need to check "Allow access to file URLs" on the extension settings.

This should not be on by default because to enable this toggle means that the extension has access to any file on your computer. For security reasons we shouldn't force that into all users, but the ones that do need this feature, should know how to get it enable.

I suggest we add more info to README regarding this and how to enable it.