Add platform_settings Runtime directive

* Creates a way for platforms instantiating this Runtime to pass
  configuration directives to blocks via self.runtime.platform_settings
  member.
* platform_settings is a dictionary of values; every block gets
  instantiated with the same platform_settings on a particular platform,
  so settings intended for a particular XBlock should be namespaced to
  it in some way.

This is needed as part of the TrackChanges PR at https://github.com/edx/edx-ora2/pull/620. Combined with an appropriate change to edx-platform (PR # forthcoming as soon as I issue it), this enables platforms to have arbitrary configuration that they pass through to their XBlocks. Without that change, this does nothing.

I'm not sure who to tag for review.

@jbau FYI.