[PATCH 00/11] Improve charset and cid: handling

[Austin Clements]

I set out to quickly add support for cid: links in the shr renderer
and wound up making our charset handling more robust and rewriting our
content-ID handling.  The test introduced in patch 2 passes in all but
one really obscure case, but only because of many unwritten and
potentially fragile assumptions that Emacs and the CLI make about each
other.

The first three patches could reasonably go in to 0.18.  The rest of
this series is certainly post-0.18, but I didn't want to lose track of
it.

This series comes in three stages.  Each depends on the earlier ones,
but each prefix makes sense on its own and could be pushed without the
later stages.

Patch 1 is a simple clean up patch.

Patches 2 through 7 robust-ify our charset handling in Emacs, mostly
by splitting the broken `notmuch-get-bodypart-content' API into
`notmuch-get-bodypart-binary' and `notmuch-get-bodypart-text' so a
caller can explicitly convey their requirements.

The remaining patches improve our content-ID handling and add support
for cid: links for shr.