Toggle comment on current line easily.

This functionality is present in almost every modern editors and IDEs, textmate, sublime for example, the conventional key binding is <kbd>s-/</kbd>. It's so ubiquitous that online websites like JSBin, JSFiddle has this shortcut build in their code editors. I have brought this up in #332 and recently someone else argues the same #600.

Since <kbd>s-/</kbd> is already bound to `hippie-expand`, I think <kbd>s-;</kbd> is also a good key binding, it's quite similar to the binding of `comment-dwim`  <kbd>M-;</kbd>.