Add a way to format Dragon's dictation words using the spoken form

[drmfinlay]

Dragon provides us with written and spoken forms for words. It would be nice if the spoken form of dictated words could be used sometimes. A good use case is where one wishes to type words like "period", which Dragon and Dragonfly both normally output as a dot (".").

An optional in_spoken_form parameter could be added to the appropriate formatting methods and documented.

There is actually a Dragon built-in rule for this — the dgnwords rule. Unfortunately, this rule stopped working properly some while ago, as did the dgnletters rule.