Custom Function Infrastructure

This starts the process of providing default Custom Functions to the compiler

RegisterHandler allows adhoc code to add itself to the runtime.  This could be used to make custom function 'packages' akin to image.Image that allows explicit adding of the jpg, png, and gif packages.

libsass function lists are now supported.  This is done by creating slices in Go and passing the address to libsass.  This code LEAKS MEMORY and needs to be freed in the compiler.