generate command now supports Timestamp based versions

[dekiftw]

Hello!

• Type: new feature
• Link to issue: https://github.com/phalcon/migrations/issues/144

In raising this pull request, I confirm the following (please check boxes):

• [x] I have read and understood the Contributing Guidelines
• [x] I have checked that another pull request for this purpose does not exist
• [ ] I wrote some tests for this PR

Small description of change: Add support for timestamp versioning when running phalcon-migrations generate

Thanks