List annex B features as optional on non-browsers

Related to the discussion in #178, which asserts that the red colour for "No" entries in the Traceur/EJS/etc. columns is misleading for Annex B features.... here's a proposal that changes Annex B rows so that the results for non-browsers are de-emphasised, signifying that they are optional. The cells are drawn in gray, and a title attribute states that "This feature is optional on non-browser platforms."

![](http://i.imgur.com/DKfsZBi.png)

They still say "Yes" and "No", purely to allow the data to remain, but it isn't expressed as important to ES6 compatibility.
Well, the build script runs in Node, not the browser.