Feature request: Add Blacklist.

[michaelachmann]

Since my shortening service is becoming more popular the amount of spammer making use of my service increased as well. In order to lock spammers out it would be perfect to have a domain blacklist - URLs on these domains will not be shortened anymore.

In the LinkHelper there's already some hard coded URLs which the link is checked against before being shortened. This function could be extended in order to implement a function on the admin interface to add blacklisted URLs.

[klezm]

I implemented the feature to implement a whitelist and a blacklist. Feel free to test and improve.
PR: #477

[draft1]

Waiting for this feature to get implemented. Thanks!