Change default zoom_level on Matplotlib's WMTS

[ahuang11]

If users ran gvts.OSM() on a global extent with the original zoom=8, it'll fetch many many tiles, and probably gets the user rate limited.

I tried to use the utils.zoom_level to auto determine zoom, but I couldn't find a way to extract the bounds from WMTS.

import geoviews as gv
from geoviews import opts, tile_sources as gvts

gv.extension('matplotlib')

gvts.OSM()

[hoxbro]

As one of those people who were hit by an API limit, this is very much appreciated.