Multi-file support

[GollyTicker]

Currently, the request and response messages need to be in the same file. An improvement would be to allow the user to import a directory of Protobuf files (or multiple directories) and have protoCURL search for the definitions given the request and response types.

In fact, protoc accepts multiple files as input directly and is not limited to a single one.

• [ ] decide how the user interface should look like (needs concrete usecase)
• [ ] write tests for this kind of usage
• [ ] implementation in Go
• [ ] add to documenation and EXAMPLES.md

[GollyTicker]

Partially implemented in #11, since protoc allows for multiple proto files as arguments.

However, having the request and response message in different files is not explicitly tested yet.