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.
Won't work in pre-ES6 filter-less environments, but not sure we care...