diff-highlight: split code into module

The diff-so-fancy project is also written in perl, and most
of its users pipe diffs through both diff-highlight and
diff-so-fancy. It would be nice if this could be done in a
single script. So let's pull most of diff-highlight's code
into its own module which can be used by diff-so-fancy.

In addition, we'll abstract a few basic items like reading
from stdio so that a script using the module can do more
processing before or after diff-highlight handles the lines.
See the README update for more details.

One small downside is that the diff-highlight script must
now be built using the Makefile. There are ways around this,
but it quickly gets into perl arcana. Let's go with the
simple solution. As a bonus, our Makefile now respects the
PERL_PATH variable if it is set.

Signed-off-by: Jeff King <peff@peff.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

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