Any interest in plain text assertion?

@dareid Do you see any value in adding a `plainText()` assertion? It would behave similar to the existing `json()` assertion. For example:

```
expect(response).to.have.plainText("chakram is awesome");
expect(response).to.comprise.of.plainText("chakram is awesome");
```

Figured I would post here before submitting a PR. Let me know what you think.

Start by locating the existing json() assertion and the assertion chains shown in the issue. Compare their behavior with the proposed plainText() examples; done means both plainText() forms assert the response text as described.

Written by the indexing model from the issue text.