vim: fix (hack) cig/cit parsing within multipart/*

vim: fix (hack) cig/cit parsing within multipart/*

The vim front-end isn't written to handle nested parts.

This patch doesn't change that, it just changes the code to pretend that
multipart/* sections end immediately. This makes the parsing code think that
all sections are top-level, and are thus parsed well enough.

The lovely result of this is that citation folds and signature folds now work
in text/plain parts that are within multipart/* sections. Also, all mime
section starts are now shown correctly (before some were not parsed and showed
the ugly ^L and an ID and so on from notmuch.)

Signed-off-by: Felipe Contreras <felipe.contreras@gmail.com>