Add a depends member to the plugin definition

Posted this on gitter but here it is as an issue.

A suggestion - add a depends member to the plugin definition that enables the import of other JS or CSS libs, e.g..:
{
"languageId": "plantuml",
"displayName": "PlantUml",
"codeMirrorMode": "",
"keybinding": "x",
"url": "https://raw.githubusercontent.com/six42/iodide-plantuml-plugin/master/src/iodide-plantuml-plugin.js",
"depends": [{
"type": "js",
"url": "https://raw.githubusercontent.com/johan/js-deflate/master/rawdeflate.js"
}],
"module": "plantuml",
"evaluator": "plantuml_img",
"pluginType": "language"
}

Start by locating how plugin definitions are read and how their URLs are loaded. Use the PlantUml example in the issue to verify that a depends member can declare JavaScript and CSS libraries and that those dependencies are imported before the plugin runs.

Written by the indexing model from the issue text.