Leave tabs transparent unless they're mixed with spaces

Currently, pretty much every single tab character has a red background. Maybe this is conventional, but I find it distracting that every line of my code started with red blocks:

![Red blocks](http://i.imgur.com/p7vqlEx.png)

I've proposed this change to highlight in red only tabs mixed with spaces:

![No more red blocks](http://i.imgur.com/1ThOKjJ.png)