Fix Rack invocation, move to Guard 2.0, and run Rack in a thread

* address issue #24 
* move from Guard 1.x to Guard 2.x
* set RACK_ENV when starting Rack
* run Rack in a thread to prevent a rogue Rack process from escaping
* offer a way to reload the Jekyllplus configuration.  E.g. `::Guard.plugin(:jekyllplus).reload_config!`