[imp] adding a new possibility to list of headers that can be filtered

[Liam Hoekenga]

> Only a few headers are supported by the imap_search function:
> http://www.php.net/imap_search

I thought that the IMAP filtering (the Ingo driver that basically replicates the
client side filtering in the current IMP release)  built into Ingo for use w/
IMP 4.x could filter based on an arbitrary header?

Liam