#33 Add command to redecode attachments with selected charset

Quoting *[Understanding email charsets by Joshua Cranmer](http://quetzalcoatal.blogspot.com/2014/03/understanding-email-charsets.html)*: > Let users override charsets when reading. On a similar token, mojibake text, while not particularly common, is common enough to make declared charsets sometimes unreliable. It's also possible that the fallback charset is wrong, so users may need to override the chosen charset.