twitter/bootstrap — comment on commit 5ad5dde1e24f668cf9b720dd59fc2a7d4808c0b3
File:  line 0
By fat:

Currently the thinking is that to generate additional columns you'll have to use the custom builder. I could also add something to the makefile if that helps you out, probably a good idea anyways.

As you can see above, adding the extra 12 columns is a lot of extra output we'd like to avoid.