Usability enhancements, Part 1 (issue #75)

This PR hides the columns of obsolete browsers by default (as per issue #75) in the ES6 compat table only. 

The changes work as expected in current versions of the major browsers. Does not work in IE <10 (but fails gracefully resp. does not change the current situation where the ES6 page is already visually broken in IE 9 and older). 

Let me know if I should alter this proposal, and/or apply the same functionality to the other tables, too. 

The code will get some refactoring in subsequent PRs.