Validation API for input files

This is not the same as an [API for validating data types](https://github.com/daisy/pipeline-framework/issues/135), although the two could possibly be combined.

It could either be
- a simple function, similar to the current [data-types API](https://github.com/daisy/pipeline/wiki/WebServiceAPI#data-types), to get the XML definition (RelaxNG) of a certain input media-type, which would allow clients to do anything you can do with a RelaxNG grammar, e.g. validation or completion.
- or it could be similar to the API described in https://github.com/daisy/pipeline-framework/issues/135 for validating data types, namely upload a file and get a validation report back. This would allow lighter clients, but is possibly more limiting.

A use case is the TTS configuration file. The new API would basically assist you with writing the config file, e.g. give you a list of supported voices. A more far-fetched use case is to validate a CSS style sheet.