add proto file's path configuration

[nethibernate]

modified the generating logic, use proto fixed dir instead of proto's parent dir;

[tcripps]

Can you please describe the goal of this change? What is the use case that it solves?

[nethibernate]

Ok, I will try to describe the scenario I have met.
Below is my proto module's structures:
proto_module
|----protos
　　　|----google
　　　　　　|----protobuf
　　　　　　　　　|----descriptor.proto
　　　|----A.proto
　　　|----B.proto
|----src

When the plugin invokes the protoc to compile all the proto files, it should specifies the parameter "--proto_path", so the "proto_path" of descriptor.proto above is "proto_module/protos/google/protobuf/". Using this "proto_path" will cause protobuf's security check exception when I run the program and the program wants to initialize any generated proto class.

This pull request added a text box to let users specify a fixed root path where all the proto files are. When the plugin compiles the files, it will use the specified path for every proto file instead of the proto file's own path. In this case, it won't cause any security check exception.