cmd/go: add join template function.
cmd/go: add join template function.

It's common to use the go list command in shell scripts, but
currently it's awkward to print a string slice from the Package
type in a way that's easily parseable by the shell.  For example:

        go list -f '{{range .Deps}}{{.}}
        {{end}}'

(and even that prints an unwanted new line at the end|).

To make this easier, this CL adds a "join" function to the
format template.

        go list -f '{{join .Deps "\n"}}'

R=rsc, dsymonds, minux.ma, remyoudompheng, r
CC=golang-dev
http://codereview.appspot.com/6680044

h8liu/golang,h8liu/golang,h8liu/golang,h8liu/golang,h8liu/golang,h8liu/golang,h8liu/golang