Improving face customization in notmuch.el

[Matt Armstrong]

Hi folks, this is my first contribution to the project.  I'm new to
the git workflow, so go easy on me.  I also know next to nothing about
elisp.  ;-)

I work for Google, which strongly supports employees contributing to
OSS projects.  Some minor legal red tape must occur because it must be
clear that Google is the legal entity that owns whatever rights (and
liabilities!) exists for the work I do.  So, I use
marmstrong at google.com and not my personal mail, and the contributor
line should say Google and not "Matt Armstrong".  Of course, given
this is a GPL project, those rights are limited and well defined.

I can, if needed, supply whatever "Contributor License Agreement" is
needed if notmuch maintains these (as the FSF does with what they
typically call "papers").

My goal here is to make the out-of-box experience better for
notmuch.el users with respect to the choice of faces.  I find that on
terminals with few colors the fonts used by notmuch are often bad
(e.g. black text on a black background).  I had a heck of a time
figuring out how some of these face choices were made.  I want to both
improve the default faces and make them easier to discover and
customize.

Trivia: I wrote https://github.com/matta/rubymail 15 years ago before
I joined Google.  RubyMail was used by "sup" after sup's author
interned at Google and learned the power of the tags+threading+search
email model.  And then, sup inspired notmuch.  If you squint hard
enough you could say I'm already a notmuch contributor, it is just
that all the code I wrote has been deleted.  ;-)