3.0.0

• Add custom option to mangle names (modules, fields and enums) to
  more Ocaml idiomatic names (snake_cased)
• Change type of deserialize error type to be an lower bound polymorphic variant
• Rewrite type mapping to ensure that no name clashes can exist.
• Fix bug in nested cursive types referencing wrong types
• Add custom options, so options to ocaml_protoc_plugin can be
  embedded in .proto files
• Support extensions
• Allow use of message name Ocaml_protoc_plugin
• *Do not treat oneof fields as required, adding a `not_set variant
  to all oneofs.
• Avoid name clash with imported .proto files
• Avoid eager evaluation of members of recursivbe modules to fix
  bug triggered in bucklescript - @wokalski