add support for parsing the imap "bodystructure" extensible form

not generating it yet from imapserver because we don't have content-md5
available. we could send "nil" instead of any actual content-md5 header (and
probably no contemporary messages include a content-md5 header), but it would
not be correct. if no known clients have problems in practice with absent
extensible data, it's better to just leave the bodystructure as is, with
extensible data.

for issue #217 by danieleggert