emacs: search archive tweak

notmuch-search-archive-thread moves to the next line after tagging. In
the normal case this makes sense, but if the region is active, it tags
the whole region and then it doesn't really. Thus only move to the
next line if region is not active.
Add explicit license information

It's nice to have for situations where this script is found outside of
the notmuch source repository (e.g. after being installed).
Add a docstring describing the program

Prefer a docstring to a header comment so we can use it as the
ArgumentParser description (formatted with 'nmbug-status --help').
Script readers still have it near the top of the file.  Since it's a
docstring, use PEP 257's summary-line-and-body format [1].

[1]: http://legacy.python.org/dev/peps/pep-0257/#multi-line-docstrings
Move the generated date from the top of the page to the footer.

It's useful reference information, but anyone who wants it will look
for and find it.  We don't need this front-and-center.  Follow the
pattern set by our header template with a triple-quoted string.

The gray <hr> styling is less agressive.  IE uses 'color' for drawing
the rule, while Gecko and Opera use the border or 'background-color'
[1].

[1]: https://bugzilla.mozilla.org/show_bug.cgi?id=239386
nmbug-status: Factor out header/footer context into a shared dict

Rather than splitting this context into header-only and footer-only
groups, just dump it all in a shared dict.  This will make it easier
to eventually split the header/footer templates out of this script
(e.g. if we want to load them from the config file).
nmbug-status: Add the time to the footer's build-date

Our repository [1] has a post-update hook that rebuilds the status
page after each push.  Since that may happen several times a day, we
might as well show the build time (as well as the date) in the footer.

The trailing 'Z' is the ISO 8601 designator for UTC.  Now that we're
showing times, it's nice to be explicit about the timezone we're
using.

The rename from date -> datetime gives us backward-compatibility for
folks that *do* only want the date.  We keep the old date formatting
to support those folks.

[1]: http://nmbug.tethera.net/git/nmbug-tags.git