Feat(plugins): basic tools for adding plugins

Just enough to make the ngHint plugin
Do we think that plugins will require that the test framework be selected before running? They should be fairly agnostic to the framework (although this might present some issues for adding a test at the end, e.g.). In that case, we could refactor this like:

```
//2
}).then(function() {
  return driver.manage().timeouts()
      .setScriptTimeout(self.config_.allScriptsTimeout);
  // 3) Set up globals and plugins 
}).then(function() {
  self.setupGlobals_.bind(self, driver);
  return plugins.setUp();
  // 4) Execute test cases
}).then(function() {
  // unchanged stuff here
```