Open preview in opposite pane to where I'm working

[axefrog]

Being on a large monitor, I like to have my preview in the left pane, because then the code's left margin is directly in front of me, rather than off to the left. My feature requests:

• Option to split the preview into a left pane by default, rather than a right pane
• Option to open the preview in the opposite pane to that in which I'm working, for cases where I already have a left/right pane open

[JaredReisinger]

Note that #368 no longer addresses a "left" setting (and it never addressed "opposite").