Rework Testing Harness into a Framework

[DavePearce]

This is necessary so that other plugins (e.g. Boogie) can use it. What must it support?

1. (Multi-Stage) Its got to support compiling Whiley files, then doing other stuff (e.g. generating JavaScript).
2. (Context) It needs to provide context for the various stages, such as what the relevant directory is.
3. (Filtering) It needs to support filtering on error code types.
4. (Error Handling) It needs to allow error handling (e.g. if compiling to WyilFile fails then we're done).