Add `meta` to template output.

[rwjblue]

Adds buildMeta callback to compile options.

If a buildMeta function is present in the options passed to compile or compileSpec, the return value will be used for the meta key in the template output.

This allows the host frameworks compiler to add metadata details (moduleName, revision, etc) as needed.

var template = compile('Hi, {{name}}!', {
  buildMeta: function(program) {
    return {
      isHTMLBars: true,
      revision: 'WAT@LOL.1234',
      loc: program.loc
    };
  }
});

template.meta.revision; // => 'WAT@LOL.1234'

---

Sample template output (no buildMeta callback):

(function() {
  return {
    meta: {},
    arity: 0,
    cachedFragment: null,
    hasRendered: false,
    buildFragment: function buildFragment(dom) {
      var el0 = dom.createDocumentFragment();
      var el1 = dom.createTextNode("Howdy ");
      dom.appendChild(el0, el1);
      var el1 = dom.createComment("");
      dom.appendChild(el0, el1);
      return el0;
    },
    buildRenderNodes: function buildRenderNodes(dom, fragment, contextualElement) {
      var morphs = new Array(1);
      morphs[0] = dom.createMorphAt(fragment,1,1,contextualElement);
      dom.insertBoundary(fragment, null);
      return morphs;
    },
    statements: [
      ["content","name"]
    ],
    locals: [],
    templates: []
  };
}())

Sample output with a general buildMeta as Ember might use it:

(function() {
  return {
    meta: {
      "isHTMLBars": true,
      "revision": "SOME@REVISION",
      "moduleName": "my-app-name/templates/foo",
      "loc": {"source":null,"start":{"line":1,"column":0},"end":{"line":1,"column":14}}
    },
    arity: 0,
    cachedFragment: null,
    hasRendered: false,
    buildFragment: function buildFragment(dom) {
      var el0 = dom.createDocumentFragment();
      var el1 = dom.createTextNode("Howdy ");
      dom.appendChild(el0, el1);
      var el1 = dom.createComment("");
      dom.appendChild(el0, el1);
      return el0;
    },
    buildRenderNodes: function buildRenderNodes(dom, fragment, contextualElement) {
      var morphs = new Array(1);
      morphs[0] = dom.createMorphAt(fragment,1,1,contextualElement);
      dom.insertBoundary(fragment, null);
      return morphs;
    },
    statements: [
      ["content","name"]
    ],
    locals: [],
    templates: []
  };
}())

[rwjblue]

@mmun suggested that we should not hijack loc.source for a non-absolute file path (and I agree). Will be updating shortly...

[rwjblue]

Completely refactored based on @mmun's feedback. Now it accepts a buildMeta callback function in options, and puts all information in meta on the template object.

[rwjblue]

Updated, to pass the program through to the buildMeta callback and remove loc from default meta.

[rwjblue]

Updated to remove some string gymnastics from the TemplateCompiler (rely on JSON.stringify for that).

[mmun]

:heart: