Support ES6 modules

[panuhorsmalahti]

Modules don't allow exporting the whole module as a function in ES6:

microsoft/TypeScript#5867 (comment)

My suggestion for a fix is to export the function from the module instead of exporting the function as the module.

[bcomnes]

Does this module not work with a specific es module loader? Do they not translate this into:

import simplifyGeojson, {simplify} from 'simplify-geojson'

?