CodeMirror: ProtoBuf mode

## ProtoBuf mode

package addressbook; message Address { required string street = 1; required string postCode = 2; } message PhoneNumber { required string number = 1; } message Person { optional int32 id = 1; required string name = 2; required string surname = 3; optional Address address = 4; repeated PhoneNumber phoneNumbers = 5; optional uint32 age = 6; repeated uint32 favouriteNumbers = 7; optional string license = 8; enum Gender { MALE = 0; FEMALE = 1; } optional Gender gender = 9; optional fixed64 lastUpdate = 10; required bool deleted = 11 [default = false]; }

MIME types defined: `text/x-protobuf`.