Make option syntax consistent with Dgraph Zero/Dgraph Server

[danielmai]

This feedback comes from https://discuss.dgraph.io/t/whats-the-correct-syntax-for-parameters/3358/2.

Dgraph Zero and Dgraph Server uses Cobra which provides conventional double-hyphen options (e.g., --addr vs -addr and a command summary.

Would be nice to replace pkg/flag with Cobra for consistency.