Step To Support Docstring Format for Filling Forms

Fixes #29 . This adds a step that will allow you to fill a text field with content specified as a doc string, e.g.:

```
I fill in "Colours" with:
  """
  Green
  Blue
  """
```

None of the other action steps seem to have tests, so I didn't do a test for this one. Let me know if I should.