Add built-in identity provider, support for others

- Update README with identity provider info
- Add syntactic sugar for default identity provider (no provider name, ie <(...)> becomes identity<(...)>
Extend the ability to pass references to providers

- Allow non-nested references anywhere within a provider call
- Allow as many of them as you want (as long as they aren't nested)
- Add documentation
- Add test
Skip references inside of double-quoted strings
Merge pull request #88 from ChessCom/MD/provider-reference-passing

Provider reference passing update