[resource-hints] spec draft update(s)

First off, big thanks to everyone for the great feedback on first draft!

There are still a number of issues to address [1], but I want to highlight
some of the bigger changes made so far and see if anyone has other feedback
or suggestions:

- hint priority is communicated via context attribute, instead of type -
see [2].
- prefetch+prerender have been merged into one hint (prerender) - see [3].
- preload is non-blocking by default - see [4].
- mechanism to indicate hint priority (required vs. speculative execution)
- see [5].
- lots of other terminology and language tweaks...

Latest draft: https://igrigorik.github.io/resource-hints/

ig

[1] https://github.com/igrigorik/resource-hints/issues
[2] https://github.com/igrigorik/resource-hints/issues/6
[3] https://github.com/igrigorik/resource-hints/issues/2
[4]
https://github.com/igrigorik/resource-hints/commit/c06fcb6596368835d60f51e66204a13acd878cab
[5] https://github.com/igrigorik/resource-hints/issues/17