Added default features

[philippjfr]

Adds default features with same default styling for bokeh.

import geoviews.feature as gf

%%output backend='matplotlib' size=200
%%opts Overlay [projection=ccrs.GOOGLE_MERCATOR]
gf.ocean * gf.land * gf.coastline * gf.borders * gf.rivers * gf.lakes

%%output backend='bokeh' size=125
%%opts Overlay [xaxis=None yaxis=None show_grid=False]
gf.ocean * gf.land * gf.coastline * gf.borders * gf.rivers * gf.lakes

[jbednar]

Looks great! I'll merge it in the morning if you haven't yet.

[philippjfr]

Ready to merge.