Script filters are executed last

This forces script filters to be executed after all other filters.  This
is really useful for regex insource filters because those have to load
every document they process.  For example,
insource:"test" insource:/test stuff/
will be faster now because the test filter will execute before the regex
filter.

Bug: 67418
Change-Id: I07d4392e5162a50db5ea3b2720cfc2d8c736ecca