hvplot accessor?

[rsignell-usgs]

As mentioned by @dcherian in #101 (comment), it would be cool to have an accessor for hvplot, which would hopefully identify lon/lat coordinates in hvplot removing the need to enter them manually if geo=True: https://discourse.holoviz.org/t/use-cf-xarray-to-identify-lon-lat/3115

[dcherian]

👍🏾 I tried to wrap accessors in general but ran into weird errors. I think we can just special case hvplot as a quick fix.