Add -t/--target flags to the other subcommands

[tsandall]

It would be nice to be able to exercise the new wasm runtime through subcommands like opa test and opa eval without having to opa build beforehand. I could imagine adding -t/--target flags to bench, eval, and test. For opa run, I could imagine either adding the same flag or allowing users to toggle the execution target in the REPL and via the HTTP API.

In terms of implementation, if we extend the rego package with an option to compile to wasm, the changes required in the subcommands would be minimal (i.e., they would just have to pipe the target flag down to the rego.New() call.)

Since we don't yet support linking between policy.wasm files, we would need to disallow -t when loading bundles containing wasm compiled policies.

[tsandall]

We might need the logic from the compile package that is used by opa build to compile rego into wasm: https://github.com/open-policy-agent/opa/blob/master/compile/compile.go#L361

I don't think we can depend on the compile package inside the rego package because that would create a circular dependency.