Improve flexibility of parsing tool

[PaulSonOfLars]

The parser reports a lot of protos as broken due to protoc being more flexible and forgiving than the official spec suggests. This PR adds a few of these edge cases, such as:

• Nested support of options

          additional_bindings {
              get: "/v1/users/{user_id}/messages/{message_id}"
          }

• Alternative syntax for nested options

          additional_bindings: {
              get: "/v1/users/{user_id}/messages/{message_id}"
          };

• Trailing commas in fields

  option (field.fields) = {
    foo: "data",
    bar: "data2",
  };

Let me know if any of these require extra data. I will be adding tests for these shortly.

[PaulSonOfLars]

This addresses the following issues: https://github.com/yoheimuta/protolint/issues/108 https://github.com/yoheimuta/protolint/issues/109

Tested locally on various problematic protos (thank god for gomodules!)