Add zoom_adjust to add_basemap

[darribas]

It'd be great to give the flexibility that zoom_adjust provides in the Place API:

contextily/contextily/place.py

Lines 36 to 38 in 5b560f9

	zoom_adjust : int or None
	[Optional. Default: None]
	The amount to adjust a chosen zoom level if it is chosen automatically.

to add_basemap. That way, the user could fine tune basemaps more easily.

[martinfleis]

Closed by #228