Add domain searching on options page

[rcutmore]

This fixes #715 by adding a search box to the options page. As user types in search box the list of displayed tracking domains is filtered.

When there is a larger number of tracking domains the first couple keystrokes are a bit delayed until the list of domains is filtered a bit. If you prefer I can update this so no filtering is initiated until a button is clicked.

[cooperq]

This looks excellent! I wonder if waiting until the search was 2 characters to start filtering would solve the key delay problem? Excellent work though, I am happy to merge this now!