Bootleg.{UI, Config}, others should be available in Tasks

Currently if the user wants to make use of `Bootleg.UI` or `Bootleg.Config` they need to import it themselves:
```elixir
use Bootleg.DSL

task :foo do
  alias Bootleg.UI
  UI.info("Ok, getting started")
end
```

We could import one or more of these modules as a convenience.
