This is a followup to issue #548, which was caused by edges being
dropped. In that context dropping edge was not really desirable, so
let's make this behavior configurable.
* spot/parseaut/public.hh: Add a new option.
* python/spot/__init__.py: Likewise.
* spot/parseaut/parseaut.yy: Honor that option.
* tests/python/parsetgba.py: Add a short test for it.
* NEWS: Mention it.