--default-names for coreutils

I realize this option is probably left out by design, but please consider:

 * The google completion for "homebrew coreutils" is "prefix"
 * The results show multiple blogposts and stackoverflow posts showing many multiple ways to use gnu-coreutils by default (despite the `brew info` instructions)

I hope this demonstrates there's a sizeable community of consenting adults that desire this option.
I understand that it *might* cause various bizarre issues, but for what it's worth i've been using this for multiple months, and in general it's decreased the number of problems I've had.

If there are still qualms, please add some kind of warning rather than rejecting my patch.


![screen shot 2014-09-11 at 9 52 52 am](https://cloud.githubusercontent.com/assets/640328/4238066/99257abe-39d4-11e4-8198-0f7da0a7d969.png)

PS: Thanks for homebrew! It's great.