Add a config option to avoid passing include_source_info

[kgrech]

Fixes #1213

We already support passing disable_comments to disable comment processing during the code generation.

However, when we compile FileDescriptorSet, we always pass --include_source_info. These might cause a problem, especially when the file is later included into the final binary using include_bytes! e.g. because of the following:

• The comments might contain sensitive private information (comments + file location) which we don't want to be visible in the binary (e.g. when someone disassemble it).
• It increases the size of the binary

Therefore, adding the skip_source_info argument allowing to disable it. Altnerative fix could be to add it protoc_args by default, but this is not backwards compatible, because protoc_args are currently additive only.