Release 3-0-0-rc2

This is my proposal for a rc2 release of Liquid 3. Once we know that Jekyll is good with this, we can publish it as 3-0-0. I want to get that out of the door in the next few days or so.

This includes everything that is on master right now, except the recent parser changes.

The diff shown in this PR is a diff against the current 3.0.0rc1 release. Diff to master: https://github.com/Shopify/liquid/compare/master...3-0-0-rc2.

I didn't bother updating the History.md file since we suck at doing that anyway and I'm kinda tired of trying to keep it up-to-date. Starting to think that that file is useless anyway.

@dylanahsmith @arthurnn @pushrax @xthexder @jasonhl 

@parkr, https://github.com/jekyll/jekyll/pull/3002 seems to pass tests with this branch. Rebased that Jekyll branch on latest Jekyll master also passes tests with this Liquid branch.