v0.5.0

Added

• Use github.com/pkg/errors to decorate errors with stack traces (thanks to
  dcomier.)
• Several improvements to Content-Type header decoding (thanks to dcormier.)
• File modification date to encode/decode (thanks to dann7387.)
• Handle non-delimited address lists (thanks to requaos.)
• RFC-2047 attribute name deocding (thanks to requaos.)

Fixed

• Only detect charset on text/* parts (thanks to dcormier.)
• Stop adding extra newline during encode (thanks to dann7387.)
• Math bug in selecting QP or base64 encoding (thanks to dann7387.)