CLI for preprocessing svelte files

[kaisermann]

Is your feature request related to a problem? Please describe.
Tries to tackle sveltejs/component-template#8 and #255.

Describe the solution you'd like
A tiny CLI (possibly using sade) to preprocess invalid .svelte files, which are files written in languages other than JS, CSS, and HTML, into valid .svelte files. This is especially useful for library authors that want to write their components using some custom language such as SASS or TypeScript.

$ npx svelte-preprocess --src=/ --out=dist/

Describe alternatives you've considered

sveltejs/component-template#31