Ref https://github.com/svg/svgo/issues/1579
In config of course. Projects with type:module can now use modules to
export config
```js
export default {
plugins: []
}
```
Also added support for resolving svgo.config.mjs and svgo.config.cjs.
Moved loadConfig tests to svgo-node tests.
mjs test is skipped for now in node 10, just don't use modules there
