Option to ignore tagged fields

This release includes a new CLI flag which causes the converter to omit specifically tagged fields.

There are two parts to this:

• Use the included options.proto and mark fields to be ignored (see example)
• Use the exclude_ignored_fields option with your protoc command (see the Makefile for a sample)

string hidden1 = 3 [(protoc.gen.jsonschema.ignore) = true];