Add context menu (Fixes GH-4)

Changes:
  - The tab click event is now a mousedown event, which gives the
    ability to detect a right click, and the tab will display just
    slightly faster now. (This is how ST does it)
  - The context menu adds "Close Tab", "Close Other Tabs", and "Close
    Tabs to the Right"
Merge upstream
Makes changes per @kevinsawicki

* Deconstruct event paramaters
* Scope `right-clicked` class removal
* Use `?=` operator in Coffeescript to check if tab is null
* Check `classList` property on mousedown target for .close-icon
Merge pull request #16 from remixz/ctx-menu

Add context menu (Fixes GH-4)