Move compiler options under compilerOptions

[benmccann]

This updates it to match rollup-plugin-svelte and also simplifies the code

[benmccann]

@non25 @Smittyvb let me know what you think about this one

[non25]

@benmccann I think it is reasonable to unify configuration interface. 3.0.0 will be breaking anyway.
And knowing that rollup is more popular, less popular solutions should mimic its behavior.
We should not forget to update template-webpack once we've done moving code around.

I kinda like the conciseness of one less level of config indentation though.

[non25]

Also it is more clear when compilerOptions are its own thing. So its harder for problems like #85 to pop up.