Setup for proposal status page

[jtbandes]

The PR for the gh-pages branch should be merged at roughly the same time as this one. (I haven't created that PR yet, because there is currently no gh-pages branch. Someone will need to create it.)

• New source of truth for proposal summary and status: proposals.xml
• Status page rendering: proposal-status.xsl

[jtbandes]

My branch https://github.com/jtbandes/swift-evolution/tree/gh-pages-new should be pushed to gh-pages in this repo.

git remote add jtbandes git@github.com:jtbandes/swift-evolution.git
git fetch jtbandes
git push origin jtbandes/gh-pages-new:gh-pages

cc @DougGregor @lattner

[jtbandes]

I'll keep rebasing this as master changes, but naturally the sooner it's merged the less pain there'll be for all of us 😉