Add query flags to mym.h
Add bigint_to_double flag to query syntax. Addresses #6

 * If the flag is set, all bigint columns will be returned as double.
   Thus, NULL values will be returned as NaN
 * If the flag is not set, bigint columns are returned as (u)int64.
   Any NULL values that are encountered will throw an error.
Add version command to mym

 * mym('version') displays version string
 * v = mym('version') returns struct with fields 'major', 'minor', 'bugfix'