Add option to set custom redirect page

[ladislavpr]

Hello, I've wanted to be able to automatically load preferred website upon block, it should work as expected.

I've actually mostly wanted to load my default new tab - which seemed to run into issues with it being blocked by an extension (possibly the extension I have on that page - TabText), so for empty string it discards tab and creates new tab, for me it works as I want, but losing tab history might not be desired.