Create meta-handler framework

[MobyGamer]

Essentially, a pre-execution handler that can do things before a title is even checked for existence. IOW, a way to set up a pre-handler to run before the regular handling.

Specific use case: Being able to set up a sort of meta-handler that uses mTCP's HTGET to download the zip file off a local media server. This allows a modern system to store multiple gigs and gigs, way more than a local filesystem could. foone in twitter had suggestions on how to proceed.