cmd/build: Refactor build command

Previously the build command was only for compiling rego into wasm and
writing the executable to disk. While that was useful for kicking the
tires, users will benfit from having a more flexible build command
that operates on bundles and supports different build-time options
like optimization. This commit addresses that use case: make it easy
to build OPA bundles from sets of policy and data files.

Fixes #1460

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