Resolve bug with mounting absolute paths to project root
Directive updates and test errors improvements.

Added support for <v:append name="">, <v:prepend name=""> and <v:replace name=""> directives as shorthand for <v:block name="" operation=""> elements. This syntax was inspired by Jade.

Added a check for any errors returned to the callback after compiling. This check now throws the errors instead of asserting they are falsey.