React developer interface [WIP]

[bvaughn]

Continues from PR #7360.

First step toward repurposing the recently-added ReactFiberErrorLogger as a more generic ReactDevInterface. I am going to abandon this PR (for now) due to time constraints but I'd like to pick it back up in the future.

The basic idea is:

• React provides a default, renderer-agnostic implementation that logs errors and warnings to the console. (This can help with things like Jest tests.)
• Specific renderers (eg DOM, Native) can inject their own UI in order to promote awareness of warnings or to help developers better locate errors.
• Application developers may choose to override renderer UI with their own hooks in order to log errors, direct users to report bugs, etc.

This PR is half-baked but it's here for posterity. 😄

UI treatment ideas for DOM

• Yellow box dialog for warnings only: https://jsfiddle.net/f8buL8Lu/
• Toast-style messages for warnings and errors: http://run.plnkr.co/plunks/dsbJMh/
• Modal dialog for warnings and errors: http://run.plnkr.co/plunks/qTLrjL/

Team feedback

• DOM and Native DX should be consistent. Doesn't have to look the same (web has more flexibility in terms of screen real estate and interactions) but should be familiar to devs and should expose the same basic information.
• Warnings and errors may be associated; don't obscure warnings as they're often more helpful in spotting problems than errors.