[cli] support for ES6

[Yzrsah]

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.<anonymous>: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