Support for buffer zone (ie. territorial waters = 12nm)

[leeprevost]

@simonepri - this was my attempt to add buffer zone logic to your is-sea module. I updated index.js and package.json but did not update readme, demos, or test. Was not sure how to do that as I'm more comfortable with python. I thought adding the @turf/buffer module would be lighter weight than trying to import another map which has buffers drawn and would be especially 'lite' for raspi or embedded use aboard a vessel. If you can guide me on whatelse needs added, I'll surely do my best to contribute.

[leeprevost]

I need to memoize/cache the buffered map to avoid recomputing it on each call.

[leeprevost]

Rather than buffer zone which requires much memory and recompute, I just refactored using @geo-maps/countries-maritime-10m boundaries.