formhandler should support groupby

Summary

Gramex FormHandler now supports _by. See gramex#401

For GET requests, ?_by=State&_by=City does the following:

• Group by State and City (in that order)
• Show sum of all numeric columns
• Show count of all non-numeric columns (including date columns)

We need following options in formhandler table

• In column dropdowns to select that column has groupby key.
• Ability to hide certain aggregate columns
• Change aggregation type of the column
• Placeholder alongside pagination to list down dimensions and metrics.

Benefits

This will allow users to create self-service explorations with formhandler interface.