BibIndex: new index 'filetype'

* Creates new 'filetype' index and plugs it into bibindex.
  (closes #473)

* Creates derived field 'filetypes' in bibfield atlantis.cfg
  field definitions.

Signed-off-by: Grzegorz Szpura <grzegorz.szpura@cern.ch>
Reviewed-by: Tibor Simko <tibor.simko@cern.ch>