In openapi-diff-cli add old and new filenames to the output

[farnulfo]

When using openapi-diff-cli we need to give and filenames.
It will be cool to have those filenames in the report output.

Before:

==========================================================================
==                            API CHANGE LOG                            ==
==========================================================================
                           API Name API
--------------------------------------------------------------------------
--                            What's Changed                            --
--------------------------------------------------------------------------
...

After:

==========================================================================
==                            API CHANGE LOG                            ==
==========================================================================
                           API Name API
--------------------------------------------------------------------------
old file: xxx.json
new file: yyy.json
--------------------------------------------------------------------------
--                            What's Changed                            --
--------------------------------------------------------------------------
...