expose umd build on npmcdn

[yoshuawuyts]

From Twitter - it'd be a good idea to add npmcdn support. All we need to do is create an (optimized) standalone build whenever npm publish is run that's written to the umd/ directory (ref).

This is useful for people using jsfiddle and the like. Think it's fair enough to add this as it's little to no overhead. Thanks!

[yoshuawuyts]

Example https://github.com/joshwnj/react-visibility-sensor/blob/master/package.json#L9