Add a configuration option to allow single click in sidebar to open a tab like double click behavior.

While I can see many users liking the current behavior, those coming from other editors (like TextMate) might wish to have the more familiar single-click-to-open-tab behavior available to them. I frequently find myself forgetting to double-click a file and then have it disappear when I click on another file. Also, seeing a file's content under the tab title of another file is disconcerting. Maybe the default behavior should create some sort of "preview" tab on the very left used only for the purpose of quickly viewing files?