Variable assignments in filters

[CoreyD97]

Feature description:
I want to be able to define variables (static, assignments from filter fields, using regex on filter fields) to use the variables within the filters.

Scenario example:
This would allow complex filtering operations such as identifying cross-origin requests for example:
$origin = request.headers | "Origin: (.*?)\n"; request.host != $origin
Where | could indicate a regex filtering operation on the data of the Request.Headers field.