Manage `$GOPATH` with `direnv`

http://direnv.net/

```
~ $ echo $GOPATH

~ $ cd code/tsaleh/gopath/
direnv: loading .envrc
direnv export: +GOPATH

~/code/tsaleh/gopath $ echo $GOPATH
/Users/tsaleh/code/tsaleh/gopath

~/code/tsaleh/gopath $ cat .envrc
export GOPATH=$(PWD)

~/code/tsaleh/gopath $ cd ~/
direnv: unloading
direnv export: -GOPATH

~ $ echo $GOPATH
```