Create functions that check if data 'osmable'

[Robinlovelace]

By 'osmable' I mean can easily be added to OSM, either as additional attributes (tags) on existing datasets or new elements. osmdata_check() would seem a reasonable function name but it could be something else. Also this leads to the possibility of a function to make data 'osmable', e.g. allowing as(x, "osmdata").

Previous discussion: https://github.com/r-spatial/mapedit/