BibIndex: PEP8 compliance fixes

* Improves code readability and removes many PEP8 compliance issues from
  bibindex_engine file.

Signed-off-by: Grzegorz Szpura <grzegorz.szpura@cern.ch>
Tested-by: Tibor Simko <tibor.simko@cern.ch>
BibIndex: changes in WordTable argument list

* Removes tag_to_tokenizer_map WordTable's argument.

* Introduces special tags as a new configuration option.

Signed-off-by: Grzegorz Szpura <grzegorz.szpura@cern.ch>
Tested-by: Tibor Simko <tibor.simko@cern.ch>
bibindex: new filename index

* Adds new `filename` index, field and tokeniser.  (closes #1717)

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