Add option for importing custom OSM tags

[leo-desbureaux-tellae]

Following #37, it appears that we may need additional OSM tags than those kept by osmnx (see useful_tags_way here).
An option should be added to the generate_osm_graph script and the corresponding function import_osm_graph in utils.py.

To specify additional tags to keep when extracting with osmnx, we can do this:

ox.config(useful_tags_way=ox.settings.useful_tags_way + ['cycleway'])

This is not very nice since we have to list manually all relevant tags (for instance, we may want to add 'cycleway:left' and 'cycleway:right' here). However, this seems inevitable regarding the number of tags OSM proposes.