Better esm support

[OmgImAlexis]

I noticed in #27 esm support was added but only in the cases of require = require('esm')(module);.

What doesn't seem to be supported is this.

const _require = require('esm')(module);
const myModule = _require('my-module');

@guybedford any chance you could help on this?

[OmgImAlexis]

I haven't checked but I don't believe this supports passing the options object to esm either.

For example:

const options = {};
const _require = require('esm')(module, options);
const myModule = _require('my-module');

[OmgImAlexis]

Wouldn't it make more sense to just look for and replace require('esm') while ignoring the rest?