Enable sourcemaps

[Anachron]

The merged pull request esnext/es6-module-transpiler#137 has enabled sourcemap-support:

ericf/es6-module-transpiler@7a94863

So when updating, make sure to enable it again and remove it from the readme: https://github.com/dbashford/mimosa-es6-module-transpiler#functionality

Note: The ES6 transpiler currently does not support source maps. So source maps will not be generated, nor will source maps already present (as from CoffeeScript) be honored or updated. They will not, however, be removed.