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_characters()
   (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_characters 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.