Perform vim-style editor pane splitting

Using percentage dimensions to absolutely position the panes on screen where they need to be located. Flexbox would have been nice, but unfortunately I could not work around what seem to be bugs (or at least major inconveniences) in its current implementation.

src/stdlib/underscore-extensions.coffee

@@ -4,3 +4,7 @@ _.mixin
   remove: (array, element) ->
     array.splice(array.indexOf(element), 1)
 
+  sum: (array) ->
+    sum = 0
+    sum += elt for elt in array
+    sum