cli: implement diff command

[audrium]

Create a new diff command that replicates all the functionality we have in reana-client.
It should also include all the flags and return exactly the same output as reana-client diff.

Please note that this command heavily uses coloring, so it would be better to implement #2 issue before this one.

Example:

$ reana-client diff workflow.1 workflow.2
==> Differences in workflow workspace
@@ -1 +1 @@

-{'retention_days': {'test/*.root': 5}}
+{'retention_days': {'**/*': 5, 'test/*.root': 10}}

==> Differences in workflow workspace
Binary files workflow.1/results/data.root and workflow.2/results/data.root differ