Allow user to import blacklists

[gorhill]

So that if someone stumble on a pretty good third party blacklist, they can import it.

[gorhill]

The challenge here is that an extension cannot save a file locally (importing entries in the extension storage is foolish). As I don't plan to have a server with a repository for this extension, it is not an easy fix. One way is to make use of the chrome.download API I suppose.

[gorhill]

Fixed in 4ab8531.