Support farbfeld as an intermediate format

Support farbfeld as an intermediate format

Sent now uses farbfeld[0] as an intermediate format.  A series of
filters is specified in config.h that matches file extensions to
filter programs.  The programs will convert between formats such as
png to farbfeld.  Internally in sent we do not need to worry on how
to parse png or any other format.

This also works with jpg and gif and others.  The 2ff wrapper will
use imagemagick conversion tools.  This is temporary as jpg2ff and
gif2ff will also be implemented.

To make this work, you will have to clone[0] and put png2ff and 2ff
in your PATH.

[0] http://git.2f30.org/farbfeld/