Please offer side-by-side diff for terminal output

This bug was originally reported by Paul Wise (pabs@debian.org) in Debian bug #908992:

The HTML output uses a side-by-side diff but the terminal output uses
`diff -u` style output. It would be nice to have an option for
side-by-side diffs with the terminal output. The icdiff package
contains a Python 3 module that could be used for this.