#48 extent accept syntax to other fields.

[psilva]

Extend accept/reject logic to include matching with fields other than the file name.
This is a very common need, such as for report processing, which may involve interpretation of the 'from_cluster', and 'source' headers.

one possible syntax to accept messages with the 'from_cluster' header value of 'C':

match_header from_cluster,C

which does only direct matches.

a second possibility:

accept_header from_cluster,C

Where the argument after the comma is a regexp, and the accept clause is incorporated into the normal accept/reject chain of masks, merely applying to a different field.

An alternative syntax would involve no new directive, just an option on the existing accept/reject:

accept C HEADER=from_cluster

This would avoid the use of plugins for a wide swath of needs. Other suggested syntax welcome.

[psilva]

now: https://github.com/MetPX/sarracenia/issues/36