Make editing text cells less jarring. This does not switch them to black and white and shrink/change the font. Instead it just adds line numbers without changing the height of the text cell.
Rebasing...
Merge branch 'style-text-cells' of github.com:mulesoft/api-notebook into style-text-cells

Conflicts:
	public/styles/cell-controls.styl
Remove black background of cell menu on text cells.
Merge pull request #149 from mulesoft/style-text-cells

Make editing text cells less jarring.