Make the text-to-speech chain flexible

[rhdunn]

It should be possible to specify the specific call chain of the different text analysis parts:

1. text reader -- splits the document events into words, numbers and punctuation;
2. context analysis -- identifies the type of punctuation (comma, etc.) and number (ordinal, year, etc.)
3. word stream -- converts the numbers and audible punctuation to words
4. part of speech tagger -- tags words with their associated part of speech
5. part of speech disambiguator -- resolves ambiguous part of speech tag assignments

It should be possible to build a pipeline of these and others in an arbitary order. This means:

1. Creating an abstract class:

   struct text_event_reader
   {
       virtual const text_event &event() const = 0;
       virtual bool read() = 0;
   };
   

2. Having all the analysis parts above implement the abstract class.

3. Making the classes take a std::shared_ptr<text_event_reader> instead of a std::shared_ptr<document_reader> (except for text_reader which starts the process).

4. Optionally hiding them behind a createXYZ method.

This will be a performance hit (need to measure to see how much), but it adds flexibility -- especially for using different grapheme to phoneme rules.

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