Put all sysprops and envvars in Props.
Added ignore_module_jar.

This will mainly be useful when running in JRebel. Eventually I want to make
ModuleLoader detect when its module.jar has changed out from under it and
recreate its class loader.

This will give us a poor man's hot reloading and allow us to run with
module.jar in the normal circumstance, thereby reducing the amount of "oh crap,
I changed classfiles out from under the classloader and now the editor crashed
in a weird place and I can't save this file."