Per-domain blacklist overrides

[Google Code Exporter]

There are certain domains that I'd like to block 99% of the time, but which 
must occasionally be (reluctantly) allowed for certain pages to function 
correctly. One example is google-analytics.com, which prevents certain sites 
(e.g. shadertoy.com) from working correctly if it's blocked.

Similar plugins like Ghostery seem to allow blocked sites to be enabled on a 
per-domain basis (while remaining blocked everywhere else). Would it be 
possible to add a similar feature to ScriptSafe?

repro steps:
 - go to https://www.shadertoy.com/view/XsSSRW
 - allow shadertoy.com in ScriptSafe
 - page will not render correctly.
 - (temporarily) allow google-analytics.com in ScriptSafe
 - page will (hopefully) render correctly.

Original issue reported on code.google.com by [e-mail] on 5 Sep 2014 at 12:38