1.x fix for reporter colours and streams

#### 1

Fix reporters not requiring the `colors` module required to output colours, resulting in error messages printing `undefined`.

#### 2

Also, print both error messages and debug messages to stderr instead of stdout allowing for the streams to be filtered easier. This is most useful, I think, with CSV/JSON, where you can now print debug messages and redirect stdout to a file:

```bash
$ pa11y -d -r csv example.com > foo
```

The above file now contains valid CSV where if debug messages were sent to stdout they'd be in the file.