cli/notmuch-show: support gzipped files

This drops "file" from mime_node_context and just uses a local
variable. It also uses the new gzip aware utility routines recently
added to util/gmime-extra.c. The use of gzopen / gzfile in addition is
a bit icky, but the choice is between that, and providing yet another
readline implimentation that understands GMime streams.