Consider improving extensibility

It would be incredibly useful to allow users to install their own schemas, rather than forcing all extensions to use the ext model and ExtendedArtifactType. Theoretically, users could define their own model and types:

/s-ramp/[my model]/[my type] vs. /s-ramp/ext/[my type].

This would not have to replace /ext. ExtendedArtifactType could be considered more of an "AdHocArtifactType".