suggestion: support for plain text output

Converting to plain text would be useful for constructing user messages e.g.

```
(defn validate-field [object attr]
  (when-not (object attr)
    (str (csk/->plain-text attr) " is mandatory")))

(validate-field user :work-email-address)
=> "Work email address is mandatory"
```
You'd probably need upper case, lower case, sentence case and title case.