Open source this repo

[ianks]

Given we have used this for awhile, and it is hugely valuable for creating a correct Liquid implementation. There are a number of projects out there that could benefit from it, so it's a shame to keep it private.

What are the steps needed to open source this repo?

cc: @dylanahsmith @Shopify/liquid-perf

[dylanahsmith]

In #37 I proposed adding the specs to the ruby liquid repo itself, given that it is our reference implementation.