Add export to `grpcurl`

[federicotdn]

Verb could also export to https://github.com/fullstorydev/grpcurl. Though the requests definitions in Verb are technically for HTTP/1.1, the syntax could kind of work for unary gRPC requests (not streams).

The only extra piece of data needed would be the location of the .proto file. Here's what it could look like:

* gRPC Example
:properties:
:Verb-Grpc-Proto: path/to/file/example.proto
:end:
template my-grpc-server.org:443
Foobar: Value 1234

** My Service
template /namespace.MyService

*** Get user
get /RetrieveUser

*** Add user
post /AddUser

{
    "name": "fede"
}

Here it is not clear if the user should use get or post or whatever, because technically they are all incorrect. But assuming any option is valid, the last header would be exported to:

grpcurl -H 'Foobar: Value 1234' -import-path /path/to/file -proto example.proto -d '{"name":"fede"}' namespace.MyService.AddUser

If the user tries to actually send the request via Verb, then it would need to error out in a friendly way.

[federicotdn]

Correction: it should be all POST and not GET.