emacs: notmuch-show-header-line: allow format strings and functions

emacs: notmuch-show-header-line: allow format strings and functions

If a string value is assigned to notmuch-show-header-line, it's used
as a format string to be passed passed to format-spec with `%s`
substituted by the message's subject.  If a function is given, it's
called with the subject as argument, and its return value used as
header line.

As before, t means displaying the subject and nil not using any header
line.

Signed-off-by: jao <jao@gnu.org>
Amended-by: db, docstring spelling fix