Support for parsing weak imports

[paul35621]

I use ProtoParser to detect if a file with a proto extension is really a Protocol Buffers file. Sometimes someone uses a weak import and parsing fails. It would be nice if weak imports are supported.

This is relevant parsing code is here.

I made pull request 3227 that implements the parsing. (not the linking)

Specification

import = "import" [ "weak" | "public" ] strLit ";"

A "weak" import means that the import is not required as a runtime dependency. Fields that depend on types defined in these imports can also be marked with a field option named weak.

Use of weak imports and weak field options is strongly discouraged and is supported by few target runtimes.