Feature request: get ".proto" file from URI

Often I find myself downloading a `.proto` file from the webz just to pass it to `grpcurl`, I know I can easily script it out but could probably be nice to have it as a grpcurl feature.

Example:

old workflow:

```
wget --continue https://raw.githubusercontent.com/grpc/grpc/master/src/proto/grpc/health/v1/health.proto
grpcurl -proto health.proto -plaintext 127.0.0.1:8080 grpc.health.v1.Health/Watch
```

new workflow

```
grpcurl -proto https://raw.githubusercontent.com/grpc/grpc/master/src/proto/grpc/health/v1/health.proto -plaintext 127.0.0.1:8080 grpc.health.v1.Health/Watch
```