Basic HTML email rendering

Messages with a MIME text/html MIME part will now be displayed with an
`HTML` button above them that will open another window and render the
HTML when clicked.

There is no sanitization performed, and inline attachment display is
not support.

This closes #2

conf/routes

@@ -8,6 +8,7 @@ GET     /mailbox                                Mailbox.Index
 GET     /mailbox/list/{name}                    Mailbox.List
 GET     /mailbox/show/{name}/{id}               Mailbox.Show
 GET     /mailbox/source/{name}/{id}             Mailbox.Source
+GET     /mailbox/html/{name}/{id}               Mailbox.Html
 POST    /mailbox/delete/{name}/{id}             Mailbox.Delete
 
 # Ignore favicon requests