Decode headers in reply

When headers contain non-ASCII characters, they are encoded according
to rfc2047. Nomtuch reply command emits the headers in the encoded
form, which makes them hard to read by humans who compose the reply.

For example instead of "Subject: Re: Rozlučka" one currently sees
"Subject: Re: =?iso-8859-2?q?Rozlu=E8ka?=".

This patch adds a new GMime filter which is used to decode headers to
UTF-8 and uses this filter when notmuch reply outputs headers.

Signed-off-by: Michal Sojka <sojkam1@fel.cvut.cz>