Add support for command line bundle loading

Previously OPA would only load JSON/YAML/Rego files off the command
line. With these changes, OPA will load .tar.gz files and interpret them
as bundles. This is useful if you want to test your bundles locally with
OPA without running OPA as a server, configuring it to pull down the
bundle, etc.

Also, update docs to mention that data files MUST be named data.json.

Fixes #870
Fixes #873

Signed-off-by: Torin Sandall <torinsandall@gmail.com>