[util] Add hb-diff

[util] Add hb-diff

A diff program written in Python that is more suitable for comparing
hb-shape output from different backends.  Main differences with stock
diff:

1. It outputs one line's comparison at a time, as opposed to batching
'+' lines and '-' lines.

2. It colors the part of the line that changed, taking word boundaries
into consideration.

You can pipe the colored output to 'less -r'.