Use content negotiation to determine output format

[bruth]

The order precedence is:

• Accept header e.g. Accept: application/json
• Extension on URL, e.g. /models/pcornet/v1.html

[gracebrownecodes]

It would make a lot of sense to me if rather than having a separate path for json with api/ prepended, all the file types existed at the same endpoints and could be requested either by Accept header, file extension, or url query parameter. I imagine this precedence:

• Missing all parameters returns html.
• Accept header precedes file extension precedes url query parameter.
• md and markdown are synonyms.

This would also remove the need for the current index page and likely promote the models page to the index position.

[bruth]

Yes this ticket was intended to converge the endpoints, at the least the common ones. The API itself will be eventually be self-describing (via the Link header) and have more granular and composite resources (such as #18).

[bruth]

I left off support for a file extension since I do not see it adding any value compared to the format URL parameter.