Turn the precompile script into a wrapper around a module.

A while back I wrote https://github.com/nicjansma/node-handlebars-precompiler by ripping out the precompiler script and hacking it up. (I mainly used it in Express.js projects to watch a directory, precompile everything on changes, and export to a client-side directory so I could easily share templates between client/server.) Obviously just hacking up the script in another npm package is a problematic approach because if the code diverges from Handlebars proper, someone has to be really on top of maintaining those hacks. So by moving this precompiler functionality into its own module, things like what I wrote can just be wrapped around the canonical precompiler.