Feature Request - Add centered window with padding mode

[reubenfirmin]

For large monitors, it would be nice to have a centered window with padding. Some applications (e.g. chrome with google results) don't make efficient use of horizontal space. It may be the only application you want on a particular desktop, but you may want the window geometry to be constrained.

So, for this, please add a Padded-Monocle mode, whereby there is symmetric padding on left/right, and the main window in the middle.