Load just manifests

[danpovey]

@pzelasko I notice with the librispeech example, the path of least resistance when writing scripts is to just call prepare_librispeech() which regenerates the recordings and supervisions' manifests. I think it would be nice to have a way to re-load those manifests if they are available on disk.

Either as a use_cached option, or as a separate function.

Eventually we'll have to consider whether to create a unified interface that will be dataset independent, but I think just doing it locally is fine right now.

[pzelasko]

That makes sense, will do.