Extend interface to save/load file metadata.

[twadleigh]

Possibly extend load_exr to return a Dict of metadata. Possibly add a Dict and/or keyword args to save_exr to save metadata.

Saving metadata is straightforward, but, as far as I can tell, there is no capability (in the C API, at least) to query the names of the metadata fields in a file, or the types of the corresponding values. One must know that a value of a known type exists with a known name to retrieve it through the given interface. Thus, the interface for loading metadata will be to expect an associative set (e.g., Dict) of field names and types for metadata to fetch, and to return a Dict with values for which a fetch was successful.

[twadleigh]

There is no point in touching this until the overhaul in #11 is complete (for which there is no current timeline or driving need).