Prompt users to execute the command.

[wyk9787]

After I run gimme master, I am not sure if it's just me but I see the following:

unset GOOS;
unset GOARCH;
export GOROOT='/Users/garrettwang/.gimme/versions/go';
export PATH="/Users/garrettwang/.gimme/versions/go/bin:${PATH}";
go version >&2;

export GIMME_ENV="/Users/garrettwang/.gimme/envs/gomaster.env"

I genuinely thought those commands are already executed and I am ready to go. After being puzzled why go version is not reflecting the developmental version, I realized those are the commands that I need to execute.

I think showing a prompt before those commands and / or updating the README will be nice.