[cli] support for ES6

JSS-cli can specify an export format `-e es6` but it cannot specify an import format for es6.

```
$ ./node_modules/.bin/jss convert dist/colors.js -f css
evalmachine.:1
export default
^^^^^^

SyntaxError: Unexpected token export
```

It works correctly with:
```
module.exports = {
```

So I must transpile everything with Babel first. It would be helpful if JSS-cli could work with ES6.

__Versions:__
- jss: 10.0.0-alpha.13
- jss-cli: 5.0.2