textutils: show_diff() API clean-up

* Fixes the spelling mistake in 'sufix' parameter.  (closes #1465)

* Makes the function more generic by removing the hard coded html tags.

* Adds tests to the textutils_unit_tests for show_diff() function.

* Fixes the usage of show_diff function in bibedit_engine, so it will
  work after changes in this function.

Signed-off-by: Sebastian Witowski <sebastian.witowski@cern.ch>
Tested-by: Tibor Simko <tibor.simko@cern.ch>