Brew Config: Go support

Adds a dash of Go support to the Brew Config to accompany the recent wider language support @mistydemeo weaved in.

Relates to this [comment](https://github.com/Homebrew/homebrew/issues/32014#issuecomment
-54135658).

Produces the output:

```
Go: /usr/local/bin/go => /usr/local/Cellar/go/1.3.1/libexec/bin/go
GoPath: /Users/Bob/go
```
At first, this happened:

```
==> go build -o mpdviz
src/github.com/neeee/termbox-go/api.go:5:8: cannot find package "github.com/mattn/go-runewidth" in any of:
	/usr/local/Cellar/go/1.3.1/libexec/src/pkg/github.com/mattn/go-runewidth (from $GOROOT)
	/private/tmp/mpdviz-9J2NLa/mpdviz-0.4.3/src/github.com/mattn/go-runewidth (from $GOPATH)
/usr/local/Library/Homebrew/formula.rb:554:in `block in system'
BuildError: Failed executing: go build -o mpdviz
```
And so I exited the terminal, re-opened it and ``` brew install --debug -v mpdviz ``` successfully compiled. Not sure if the initial failure relates to this PR or not.

The actual config itself doesn't throw any errors when there's no static ``` $GOPATH ``` set, It just displays:

```
Go: /usr/local/bin/go => /usr/local/Cellar/go/1.3.1/libexec/bin/go
GoPath:
```
So it doesn't find any Homebrew-created dynamic ``` $GOPATH ```, It seems to be capable of finding only statically-set ``` $GOPATH ```s.

The formula successfully compiles both with and without a global ``` $GOPATH ``` set.