lei convert: support IMAP output and "-F eml" inputs

lei convert: support IMAP output and "-F eml" inputs

eml ("message/rfc822" MIME type) is supported by "lei import",
so it probably makes sense to support via convert, at least
for tests.  And IMAP support is supported in "lei q -o $MFOLDER",
so this only required renaming {nrd} => {net} and initializing
outputs before augment preparation (creating the IMAP folder)