Extend nnimap to request Gmail labels with message headers

[Trevor Murphy <trevor.m.murphy@gmail.com>]

This is a proof-of-concept / RFC series of patches.  It may not make sense to
treat these labels as message headers, I just find it suits my workflow pretty
well.

I had to significantly rewrite `nnimap-header-parameters' to incorporate the
option, so please pay special attention to the first patch.  Note that the it
shouldn't change the functionality, it's just a rewrite that makes the later
patches possible.

After that first patch the next two just parse Google's response.  I haven't
extensively tested Google's API, so even though this code works for me and
looks simple, I don't know when it will break.

The net result is that you get X-GM-LABELS exposed as a message header.  The
content is always a parenthesized (possibly empty) list.  Full docs here:
https://developers.google.com/gmail/imap_extensions#access_to_gmail_labels_x-gm-labels

I add this to `gnus-extra-headers' and use it all over my Gmail summary
buffers.

For instance, you can display the labels in summary buffer lines with a user
format function and narrow the buffer to a label with limit-to-extra.

Thoughts?

-- 
Trevor Murphy, ASA, MAAA
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