dim: decode email message content charset to unicode

Email messages need two levels of decoding: First, content transfer
encoding, such as base64 or quoted-printable. Second, charset decoding.

We've done the first (with part.get_payload(decode=True)), but we've
ignored the charset. Mostly, it has not mattered, since most email is
ascii or utf-8 anyway, and python2 has been relaxed about it. However,
python3 part.get_payload(decode=True) gives us binary instead of
unicode, so we also need to do the charset decoding to get the result we
want.

The problem has likely been observed only now that 'python' no longer
exists or points at python3 instead of python2.

Use part.get_content_charset() for charset decoding, defaulting to
'us-ascii' source charset if nothing is specified.

Cc: Rodrigo Vivi <rodrigo.vivi@intel.com>
Cc: Daniel Vetter <daniel@ffwll.ch>
Reviewed-by: Rodrigo Vivi <rodrigo.vivi@intel.com>
Tested-by: Rodrigo Vivi <rodrigo.vivi@intel.com>
Signed-off-by: Jani Nikula <jani.nikula@intel.com>