Allow to chain add_osm_feature/s function calls

[jmaspons]

Make add_osm_feature(s) do the proper AND and OR additions to the osm feature filters

Before the patch:

• add_osm_feature collapse features from add_osm_features if called first
• if add_osm_feature is called first, features get eliminated after calling add_osm_features

After:

• add_osm_feature add features to each item from opq$features
• add_osm_features add features to the opq$features vector with new items

Bonus: features parameter now is parsed using paste_features which allow to pass more than one value for each list item

[jmaspons]

Ready for review @mpadge

[jmaspons]

All green now, @mpadge

[mpadge]

Great, thanks @jmaspons :grinning: