RFC 51: Rename lint.whitelist to lint.ignore

[zcorpan]

I support this change. But:

• The title says .ignorelist but the text says .ignore. Please pick one. :)
• It would be good to understand the impact of downstream projects better.

[foolip]

It's now lint.ignore everywhere, and I think that looks good!

[stephenmcgruer]

I have updated the RFC with the note about Gecko and Chromium; hopefully that addresses @zcorpan's concerns.