Support Syrup models at the app level

[pettazz]

Currently Syrup will only try to load a model from its own core directory inside JACKED_MODULES_ROOT, which means if any app wants to create their own models they have to either fork JACKED or keep their model files untracked from git, which sucks.

Syrup should allow for a user configurable Models search path outside of the core stuff, like the way $PATH works, with core as the first search locations then conf-defined added after.