Add --keep option to keep any remaining stuff in index or file. We
could distinguish between failures to index and failures to apply tags
or maildir sync, but for simplicity just have one.
Previously, this was a verbatim copy of the --format=json text.
Change it to instead reference the JSON text and actually describe how
the S-expression format works.
The main goal is to support gzipped output for future internal
calls (e.g. from notmuch-new) to notmuch_database_dump.
The additional dependency is not very heavy since xapian already pulls
in zlib.
We want the dump to be "atomic", in the sense that after running the
dump file is either present and complete, or not present. This avoids
certain classes of mishaps involving overwriting a good backup with a
bad or partial one.
This is the output from sphinx-quickstart, massaged a bit, along with
our existing man pages converted to rst.
A skeleton notmuch-emacs manual is also included. It is not suitable
for end user use yet.
