Setup the GMimeStream only when needed

I ran into this while looking at the vim plugin.  Vim's system() call
redirects output to a file and it was missing many of the part{ lines.

If stream_stdout is setup too early, it will overwrite the part start
when notmuch is redirected to a file.

Reviewed-by Carl Worth <cworth@cworth.org>: GMime is calling fseek
before every write to reset the FILE* to the position it believes is
correct based on the writes it has seen. Our code was getting
incorrect results because our GMime writes were interleaved with
non-GMime writes via printf.

The bug appears when writing to a file because it's seekable, but not
when writing to a pipe which is not.