Extra metadata keys

[AlbertDeFusco]

The conda-project.yml spec reads a few keys, but will ignore (without a warning) anything it doesn't require.

@jlstevens , Is there any other metadata we would want to promote in the docs or provide special handling for in conda-project?

[jlstevens]

I think we discussed having a metadata key under which people should be encouraged to add metadata.

The policy of ignoring without warning means no one has to use it, but I think it would be good to encourage people to put any arbitrary additional metadata in one place, even if it is just a convention.