Add new generation option for using proto sources from other frameworks.

- Better docs in the generator for the different options that can be passed
  during an invoke of protoc.
- Add named_framework_to_proto_path_mappings_path to pass the path to a file
  containing mappings of frameworks for different proto files.
- Update the generation to use the mapping to change the #import directives
  it creates.

Note: the changes in helpers is mostly moving code within the fine, and then
a small change to expose the parsing so a passed on class can consume the line.

Fixes https://github.com/google/protobuf/issues/1457

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