cleartext indexing, revision 7

[Daniel Kahn Gillmor]

What follows is the 7th revision of the cleartext indexing series.

As mentioned in the previous discussions, there are a handful of minor
cleanups (spelling, formatting).  Beyond those, the main differences
between this series and the 6th are:

 * changed names of variables that support command-line indexing
   arguments.  Hopefully this makes it slightly less confusing.

 * introduced documentation about which config variables are stored in
   the database and which are in the config file, which wasn't
   previously documented.

 * added some tests for "notmuch insert --try-decrypt"

 * added python bindings to enable explicit control over cleartext
   indexing (this is in the novel final patch in the series)

Thanks to Jani, bremner, jrollins, and Tomi for the earlier reviews.

       --dkg

Series History
--------------

the first version of this series was sent starting at:

id:20170912230153.4175-1-dkg at fifthhorseman.net

version 2 of this series was sent as:

id:20170915055359.24123-1-dkg at fifthhorseman.net

version 3 of this series was sent as:

id:20171010054916.23925-1-dkg at fifthhorseman.net

and version 4 was only partially sent to the list.

version 5 was sent as:

id:20171015064807.14205-1-dkg at fifthhorseman.net

version 6 was sent as:

id:20171017191008.8742-1-dkg at fifthhorseman.net