Added :env option for runner to easily set environment variables for the command

 Environment variables can be supplied throught the `:env` option.

```ruby
package :magic_beans do
  runner "make world", :env => {
    :PATH => '/this/is/my/path:$PATH'
  }
end
```

Will result in:

```bash
env PATH=/this/is/my/path:$PATH make world
```