Allow transformer to be registered at the template level

[patrick-steele-idem]

Currently, a transformer can only be associated with a tag. Certain transforms should be executed once for the entire template.

We should add support for a new transformer property within a marko.json file:

marko.json:

{
  "transformer": "./my-transformer.js"
}

Template-level transformers should be invoked once after parsing, but before traversing the tree to run transformers on HTML elements. A template-level transformer should be invoked with only one argument that is the CompilerContext:

module.exports = function transform(rootNode, context) {
};