Extension lazy loading

[abose]

Extensions should only be loaded as and when required to improve load and memory performance. To enable this, add the following configurations to the extension package.json :

{
  "toolbarIcons": {
   "domID1": ["imageURL", "hoverImage", "active image"],
   "domID2": "imageURL"
  }, // what about hover states, tooltips, translations?
  "commands" : ["command ids and registration info"], 
  "menu items": ["menu items and shortcut info"], // support loc of these strings
  "loadWhen": {
    "fileOpened": ["html", "js"],
    "codeHintsRequested": ["html", "css"],
    "eventTriggered": ["module:eventName1"],
    "domElementClicked": ["domElementId"]
  }
}

extend the current package format: https://github.com/brackets-cont/brackets/wiki/Extension-package-format