Turn on CodeMirror gutter filler

This prevents line numbers from being displayed to the left of the horizontal scrollbar (in the gutter).

Screenshot: https://f.cloud.github.com/assets/1197144/444872/e0f41e82-b1ad-11e2-89c6-7b1fbb4e0388.png

Currently, the fill color is white, which is the same as the filler on the right side of the horizontal scrollbar (below vertical scrollbar). Should this be changed to something else?

cc: @njx @larz0