Use .mjs for module build

[realityking]

This will improve compatability with Node.js

[trotzig]

We had this a (long) while back but had to revert because it broke things for a lot of people. It might be worth reading up on some discussions there:

Introduced in
#220

Reverted in
#223

There's been a lot of water under the bridge since then however, and it might be just fine to make this change today.

[realityking]

The most relevant discussion seem to be in #234 and #221

I think this might've been fixed by #286 but I'm also not confident so I'm gonna close this one.

Until react-waypoint actually adds an export map to run modules in Node it doesn't give any benefit anyway.