Add new configuration options to AssetsCompiler

New configuration options that allow the compilers in AssetsCompiler
to be configured without modifying the AssetsCompiler filer directly.
This also allows you to easily add new compilers.

This is just a first attempt. Does this seem like a good way to
configure the compilers?
simplify compiler options

Moved compiler options to compiler object.
Compiler is now picked based on file extension rather than folder.
Updated precompile to only iterate over files once
restored advanced folder options (assets/coffeescripts => public/javascripts)
Merge pull request #308 from mghayes/patch-1

Add new configuration options to AssetsCompiler