This removes --unified from the default options to make it possible to pass -y
without worrying about conflicting output formats.
diffu has also been added as an alias to make it easier to get output as a
unified diff.
Fixes#1231
The new call is compatible with colordiff v1.0.8 - v.1.0.15 (might be
compatible with even earlier versions, but not tested), while the
original one breaks down under v1.0.14 and v1.0.15. See
https://github.com/daveewart/colordiff/issues/22.
