Implement length option for time fields in MySQL

[abeverley]

SQL::Translator::Producer::MySQL specifically does not allow length fields to be used for fields containing the time, despite the length allowing the precision of fractional seconds in the underlying database (introduced in MySQL 5.6.4) - see @no_length_attr in the source code.

I am just raising the issue first and intend to follow-up with an appropriate PR at some point. Any comments welcome.

Thanks, Andy