BUGS

A pyspf query object should be usable for multiple queries.  The simple
per query DNS cache has no TTL logic, however a mail connection could be
considered a "single transaction", and the cache could be reused for
multiple SPF evaluations (HELO, multiple MAIL FROM) within a session.
PTR would especially benefit from this, and it could help justify always
doing HELO checks.

Make type SPF queries optional.  Perhaps order= option, with
order=TXT / order=SPF / order=TXT,SPF / order=SPF,TXT

"Compile" SPF records to IP sets, and cache the compiled records.

Support TCP DNS (already supported with dnspython, I believe).

type99.py should be able to handle multi-line TXT RRs (and output
multi-line type99 RRs as needed).

Improve permerror diagnostic:

email.teamfanshop.com:
  - TXT: "v=spf1 mx a:smtp1.email.teamfanshop.com ip4:12.111.131.193 /26 ~all"

DONE Customized Received-SPF header (select key-value pairs).
DONE Received-SPF header still doesn't conform to RFC4408.
DONE Collect op= modifiers as a dict of keywords.
DONE Provide 'mechanism' key-value pair in Received-SPF.
DONE Received-SPF header doesn't quote key-value-pairs where needed.
DONE Provide pluggable DNS drivers.
DONE problem keyword in Received-SPF should not have list brackets.