HTML preprosessor support

[maraisr]

I know it's very subjective - but I'm a huge advocate of Pug or JSX, anything that can help me write HTML!

Would be kind of nice to be able to write my templates with Pug, and have Svelte interpret that, whether or not it's an extra build step or not.

Example:

HelloWorld.pug

.my-app
   h1 {{hello}}

And being able to import that into a HelloWorld.js file

import HelloWorld from './HelloWorld.pug';

new HelloWorld({
   target: document.body,
   data: {
      hello: 'World'
   }
});

My only concern is writing JavaScript in Pug is terrible, so like isolate the template element to Pug and the JS, events, handlers, etc... to a JS file.

[esarbanis]

@Rich-Harris I think this is covered in #876, we can close this one.

[Conduitry]

Closing this since we have svelte.preprocess. If someone disagrees, we can reopen.