[PATCH] Display extra headers for emacs-mua - db config option

[Johan Parin]

This is another version of my attempt to get configurability on extra
headers to be displayed in the emacs MUA. It is a modification of a
previous patch where the extra headers returned by notmuch-show were
hard coded. In this version, the extra headers is configured by a
database config option `show.extra_headers'. As I see it the advantage
with this approach is:

- It gives full flexibility to choose any extra header to be
  displayed. The required steps are two (1) configure
  `show.extra_headers' in the db and (2) set `notmuch-message-headers'
  as desired.
- There is absolutely no performance concern.

The change is also very simple, low impact.

There is no API change (if that is considered a maintenance cost).

If a decision would later be taken to have format_headers_sprinter
return all headers, then users having adopted the configuration option
would still have the same functionality, albeit with an unused db config
entry.

I think this patch is complete, I also added an update to the config man
page. I guess this patch does not require updating the schemata (?)


/Johan