feat(core): support groupBy directive

[gogoout]

Hi, thanks for the query builder. Just try to implement groupBy directive as it's needed in my application. Let me know if you need any changes. Cheers.

changes:

• add groupBy in core/directive
• add a groupBy method in edge-builder
• add test
• add doc
• all directives should be lowercase, see https://dgraph.io/docs/query-language/ignorereflex-directive/ and https://dgraph.io/docs/query-language/groupby/

fix #6