Allow writing the boolean as a string in TD

[trung-huynh]

There is no boolean type in TD's table. At the moment the long type in TD's table will be created if it is boolean type in Embulk's schema and the value mappings are:

• false value will be written as 0.
• true value will be written as 1.

There is a use-case that those values should be written as strings i.e. true and false, thus a new plugin option should be added so that the boolean type could be written to either long or string depends upon the option. Default value of the option should be written to long so that it does not break anything.