Support extend definition

[yoheimuta]

It seems to be invalid according to https://developers.google.com/protocol-buffers/docs/reference/proto3-spec, but this is a common style. So I will support this definition by default.

ref.

• https://developers.google.com/protocol-buffers/docs/reference/proto2-spec#extend
• https://github.com/protocolbuffers/protobuf/issues/4610#issuecomment-388971028
• found "extend" but expected [;] · Issue #27 · yoheimuta/protolint

[yoheimuta]

Done.