Add configuration option to close the current window with or after the last tab

Fixes https://github.com/JRHeaton/atom-close-after-last-tab/issues/2 and related to https://github.com/atom/atom/issues/863 and https://github.com/atom/atom/pull/2411.

This adds a configuration option which allows users to select whether the window will be closed with or after the last tab. By default, the option is set so that the window is closed with the last tab to match the current behavior.

Also, this changes the behavior of the package so that all tabs are counted, not just editor tabs. That makes sense to me and matches my expectations, but still wanted to point it out.

cc @JRHeaton @jingchan @joernhees