Cache invalidation API

In `ts-node`, if someone modifies a `.ts` file, removes it from their `require.cache`, and then re-requires it, we will have a new sourcemap for that file. We need a way to invalidate that entry in your cache so that you re-retrieve it from us.

This issue is requesting a cache invalidation API. All we need is a function to call, passing the path of a file that you have previously requested from us, telling you to remove that entry from your cache.