add a liquid header for jekyll friendliness

This adds a liquid header to the spec markdown. The reason why this is useful is that it lets a jekyll-based site like dataprotocols.org include the spec cleanly as a git submodule, rather than needing to copy and tweak the file manually. It is a bit strange to modify a spec file to benefit a random website, so no problem if you'd rather keep the spec as pure markdown. But since the header [displays ok within github](https://github.com/paulfitz/ndjson-spec/blob/liquid/specification_draft2.md) I thought I'd at least suggest it.

I moved some version information into the header since it didn't seem to reduce readability and makes it easier to consistently format that information.

Thanks!