add timeout option to looper

[rschmukler]

reissue of #29 that actually works.

[nathany]

Thanks.

What I've been thinking of doing is making all arguments pass through. So looper <args> becomes go test ./path/to/package <args>.

[nathany]

Would #26 do what you need? I should get something like this implemented.

The idea comes from https://github.com/rsc/gt

[rschmukler]

Definitely, it'd even be more flexible than what I have implemented.