Allow #AARRGGBB color definitions to pass HexValidation

In IE filter definitions, #AARRGGBB is the way to specifiy a color
with some transparency. See:
http://msdn.microsoft.com/en-us/library/ms532879%28v=vs.85%29.aspx

Fixes #214
I wonder if this should only match if it is `=#\h{8}`, or is the MS/alpha transparency valid in any other places