Expose prototool as a Bazel rule

[jathu]

In this PR we create a rule that allows prototool to be invoked by a Bazel run command. Much of the inspiration came from buildifier.

Although we can currently invoke prototool using the provided binary, it requires all arguments to be absolute:

> bazel run @com_uber_prototool//cmd/prototool -- lint $PWD/protobuf/grpc/health/v1/health.proto
../../../../../../../../../../../../../../../../../Users/appleseed/protobuf/grpc/health/v1/health.proto:26:1:Expected "healthv1" for option "go_package" but was "google.golang.org/grpc/health/grpc_health_v1".

As you can see, the output isn't that elegant either. Compare that to the new rule:

> bazel run //:prototool -- lint protobuf/grpc/health/v1/health.proto
protobuf/grpc/health/v1/health.proto:26:1:Expected "healthv1" for option "go_package" but was "google.golang.org/grpc/health/grpc_health_v1".

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