textutils: create function to show diff view
textutils: create function to show diff view

* New function to show diff of two records

* Use that function in BibEdit's comparison of revisions

Reviewed-by: Javier Martin Montull <ea6b80e26c65acc242d8916fb6eefa01b123cb69@cern.ch>
Tested-by: Tibor Simko <c7116e45300b163846dc01f3ed02116e3f8ef7a5@cern.ch>

inveniosoftware/invenio-base,tiborsimko/invenio-base,inveniosoftware/invenio-base,tiborsimko/invenio-base