Pass dependencies from preprocessor response through in transform plugin

[Conduitry]

It would be nice for preprocessing to be able to make use of the feature available in Rollup 0.61+ that lets transforms specify additional files as dependencies, so that watching works properly.

This will be slightly fiddly as we'll have to intercept the responses from the markup/style/script callbacks to grab the dependencies from them, before passing the response along to svelte.preprocess. Then we'll compile the component (as we do now), and finally include those gathered dependencies in the response from the transform hook.

[Conduitry]

Opened PR #41. Still needs some readme updates to reflect the new feature. Probably passable docs now included.

[Rich-Harris]

closed via #41