[ENH] Modify `add_basemap` for local windowed reading about contextily

[jorisvandenbossche]

To avoid to add yet another keyword (and one that is only valid depending on the value of another keyword), we could also reuse reset_extent for this?
If reset_extent=True (default), we can read only the needed part of the raster file, as the rest is not shown on the figure anyhow.
If reset_extent=False, we of course read the full raster, as we use in this case the extent of the raster as extent of the figure.

[darribas]

GReat idea! +1

[darribas]

Addressed in #129