Custom import maps

I'm really liking the new rewrite, thanks so much for getting this moving.

Again, I'm making a pull request to the rewrite because it seems like the right way to do this, but if suitable we can get it made against the original repo.

This enables import maps to be specified for AMD and CJS compilation.

Example:

```
  compile-modules my-module.js --to output.js --imports jquery:normalized/jquery/name
```

would convert

my-module.js
```
  import 'jquery';
```

to
output.js
```
  define(['normalized/jquery/name'], function() {
  });
```

This feature would be very useful to plug in normalization maps to the transpiler, allowing custom normalization rewrites.

I did include a VERY simple test in `tests/import-map.js`. It didn't fit the existing test pattern, so I didn't want to spend too much time setting up a harness that isn't going to be used.