Added added a conversion flag to public interface

1. Added a new CangjieConversion enum to the header file
   with the below values:
   - CANGJIE_CONVERSION_OFF           = 0
   - CANGJIE_CONVERSION_TO_SIMPLIFIED = 1

2. Added a new parameter conv (``CangjieConversion``) to
   ``cangjie_get_character()``
   (interface only, feature not yet implemented)

3. Changed the benchmark and cli code to adapt the new parameter.
   Both use ``CANGJIE_CONVERSION_OFF`` only

4. Added a constant ``SIMP_QUERY`` to store a modified version of
   BASE_QUERY to query the ``chchar_simplified`` instead of chchar

5. Added a ``cj_simp_query`` to type ``Cangjie`` for the new query.

6. Added code in ``cangjie_new`` to generate this new query along.

TODO: Modify ``cangjie_get_character`` to implement the new interface.
TODO: Modify cli to add the new interface option.
TODO: Modify benchmark code to add benchmark of the new interface.