Implement multipart/alternative emails

With this commit, verification emails and messages will be delivered in
standards-compliant multipart/alternative emails in text/plain and
text/html parts, the former being charset us-ascii, and the latter being
UTF-8. In the future, messages should be parsed in markdown for the
text/html part.

Close #125
Format markdown in text/html part of messages
Merge branch 'f.multipart-alternative'