Support .mjs as file extension for es modules

...assuming they have been transpiled properly that is, like webpack does.

Example setup: https://github.com/mobxjs/mobx/pull/1131/files

Should solve facebook#59

Beyond that I have no clue what I am doing, so this is just a proposal 😁

packages/metro-bundler/src/defaults.js

@@ -43,7 +43,7 @@ exports.assetExts = [
   'ttf',
 ];
 
-exports.sourceExts = ['js', 'json'];
+exports.sourceExts = ['js', 'json', 'mjs'];
 
 exports.moduleSystem = require.resolve('./Resolver/polyfills/require.js');