BibFormat: fix multiple output format option
BibFormat: fix multiple output format option

* Enables to submit bibreformat jobs with several output formats
  in one go, for example '-o HB,HD'.  (closes #712)
* Checks the validity of the input formats at the submission time;
  if any format is invalid, the task is not submitted.
* Defines function called 'get_visible_output_formats' because
  it is used in two different places.

inveniosoftware/invenio-search,tiborsimko/invenio-records,tiborsimko/invenio-search,inveniosoftware/invenio-records,inveniosoftware/invenio-search,tiborsimko/invenio-records,tiborsimko/invenio-search,inveniosoftware/invenio-records