Sphinx extension metadata

[bjones1]

Curerntly, our Runestone Components don't return any extension metadata. We should do this! We could probably define a standard value to return for all Runestone Components, then just return that.

[bnmnetp]

Can you say some more? I looked at the page briefly but I'm not getting what problem this would solve...

[bjones1]

I can see several minor advantages:

• Specify a version, which we already have, in case anyone uses our components in some larger project.
• Specify an env_version, and increment this anytime we change what we store in env. This helps avoid confusion -- it will auto-rebuild the book when the env_version changes, instead of getting issues with crashes/confused builds.
• Specify parallel_read_safe as True. This might improve build performance a bit.

All this can go as a constant in runestonedirective.py, then each setup function returns this constant.

So, nothing critical, but an easy addition with some nice side benefits.