Adds Hide Preview Pane option
Refactor splitting into method; improve usability

Add 1:3 and 3:1 split options.

Auto-rendering is turned off automatically when preview is hidden.
Rendering is triggered when preview is visible again.

Manual rendering menu entry is disabled when preview is hidden
because it doesn't make sense.
Add Aaron Raimist as contributor
Merge branch 'preview-pane-toggle'

Close #44.