Adding support for registering new block or inline rules

I've gone through a process to allow for the addition of new unique rules to **marked**. A new rule can be added easily enough by passing a properly formatted POJO to registerRule. The downside of this addition is a reduced parse time of 32.6% when running the included bench test. The original project was focused on speed, and I wonder if this is enough deviation to be a full fork and not be pulled in.