Together with the previous patch, this Fixes #353.

Implementing this required to extend our interface two support
two-parameter patterns.

* spot/gen/formulas.cc, spot/gen/formulas.hh: Implement it.
* bin/genltl.cc: Add --sejk-f.
* bin/common_output.cc, bin/common_output.hh: Adjust to handle
"line numbers" that are not integers (e.g., "3,2"), since those
are used to display pattern parameters.
* bin/ltlfilt.cc: Adjust.
* python/spot/gen.i: Add support for two-parameters patterns.
* tests/core/genltl.test, tests/python/gen.ipynb: Augment.
* NEWS: Mention it.