Add part filename and content-id in notmuch show output if available.

Add part filename and content-id in notmuch show output if available.

Before the change, notmuch show output had filename only for
parts with "Content-Disposition: attachment".  But parts with
inline disposition may have filename as well.

The patch makes notmuch show always output filename if available,
independent of Content-Disposition.  Both JSON and text output
formats are changed.

Also, the patch adds Content-id to text output format of notmuch
show.

The main goal of these changes is to have filenames on Emacs
buttons for inline attachments.  In particular, this is very
helpful for inline patches.

Note: text format changes may require updates in clients that use
it.  The changes are:

* text part header format changed from:

    ^Lpart{ ID: 2, Content-type: text/x-diff

  to:

    ^Lpart{ ID: 2, Filename: cool-feature.patch, Content-type: text/x-diff

* attachment format changed from:

    ^Lattachment{ ID: 4, Content-type: application/octet-stream
    Attachment: data.tar.bz2 (application/octet-stream)
    Non-text part: application/octet-stream
    ^Lattachment}

  to:

    ^Lattachment{ ID: 4, Filename: data.tar.bz2, Content-type: application/octet-stream
    Non-text part: application/octet-stream
    ^Lattachment}