declarative use of alternate view compilers

I see a lot of open issues and other pull requests to incorporate other view templating languagesinto hem. I imagine one concern is the increased dependency of hem on varieties of other modules, when a given user of hem probably only uses one of them.

This adds a "compilers" map to slug.json, allowing custom packages to provide shims to the other compilers. Then the app that's using spine and wants an alternate view language can add its own dependency.

In addition to avoiding cluttering hem with dependencies, it also allows users to avoid using the slug.js convention that hem has promoted to-date.