Configurable transfer of indices and validators to history tables

• #6 Previously, all column indices were transferred to history tables. Now, certain columns can be excluded by setting them as, e.g., __chrononaut_disable_indices__ = ['col1', 'col2'] on the model.
• #9 Previously, validators were sometimes transferred to history tables and sometimes not. Copying validators over to the history tables is not a desired behavior, since validators can change over the course of time. Now, validators are not copied by default unless the __chrononaut_copy_validators__ flag is set on the model.