Specify that svelte compiler options can be specified in loader options

[AleksandrSl]

It's not obvious that compiler options such as customElement and shared can be specified in loader options. Maybe it's worth mentioning in README

[UziTech]

Wow! I have been trying to export a customElement for longer than I care to admit. If I knew all I had to do is add customElement: true to the svelte-loader options it would have saved me lots of time.

[benmccann]

The options were moved under compilerOptions in master. I agree it'd be good to add to the README. I'll review and merge a PR if anyone wants to send one