BibFormat: use run_shell_command() for snippets
BibFormat: use run_shell_command() for snippets

* When generating full-text snippets, use run_shell_command() for
  safer argument escaping.

* Also, use CFG_PATH_PDFTOTEXT.

tiborsimko/invenio-formatter,inveniosoftware/invenio-formatter,inveniosoftware/invenio-formatter,inveniosoftware/invenio-formatter,tiborsimko/invenio-formatter,tiborsimko/invenio-formatter