Support header/footer variants for javascript targets in ksc
While working on tidying up `kaitai-struct-loader`, it's dawned on me that the UMD header/footer isn't necessary within webpack.

Ideally, the output for a `ksy` being compiled for use in webpack would look like vanilla CJS:

```js
var SomeKsyImport = require('./SomeKsyImport');

function TestKsy(_io, _parent, _root) {
  this._io = _io;
  this._parent = _parent;
  this._root = _root || this;

  this.foo = new SomeKsyImport(this._io);
}

module.exports = TestKsy;
```

Perhaps we could signal this with a command line flag? `--js-module-format cjs` for example?

Another useful variant would be ES6 modules (which, ostensibly, webpack 2 can also consume):

```js
import SomeKsyImport from './SomeKsyImport';

function TestKsy(_io, _parent, _root) {
  this._io = _io;
  this._parent = _parent;
  this._root = _root || this;

  this.foo = new SomeKsyImport(this._io);
}

export default TestKsy;
```

Maybe `--js-module-format es6`?