Fix types for generate option

[JulienPradet]

Hi there 👋

I've noticed that while the generate option is documented in the plugin, it's not exposed in the Typescript's defintions.

I used the type from https://github.com/sveltejs/svelte/blob/d19bcef6901768844cc89cb39d29873381f9969a/src/compiler/interfaces.ts#L111

It would be possible to change the Options interface to extend the CompileOptions type from Svelte.

+import { CompileOptions } from "svelte/types/compiler/interfaces";

-interface Options {
+interface Options extends CompileOptions {

But it could be misleading since some options are fully handled by the plugin and can't be changed (format, sveltePath, css) and I'm not sure how well it would handle Svelte v2.
I'm definitely not a Typescript expert so I've used the safe way here!

Have a nice day!

PS: if it does not make sense to have the underlying options in the typescript definitions, here is a workaround.

declare module "rollup-plugin-svelte" {
  interface Options {
    generate?: string | false;
  }
}