Add option to turn off Autoprefixer or Enable controlling only IE hacks

[silvenon]

I always end up with my CSS already prefixed before running buildProduction, because I prefix them in development (I'm sure most people do). Because of that sometimes my prefixes get messed up because I didn't keep the options in sync. It would be awesome if there was a way to turn off Autoprefixer and only control IE hacks.

[Munter]

In programmatic usage setting the option to false disables it. I don't know if this is possible on the way through optimist in the cli wrapper

[papandreou]

There's no --autoprefixer or --no-autoprefixer switch, but I think we should add one. Please take a look at #160 before starting any work on it, though.

[papandreou]

Now that #160 was merged, this is easy and uncontroversial to add. Anyone up for it?