Feature request: Implement before and after parameters

[jotolo]

Squasher is a very helpful library to organise migrations and squashed them for instance, into one migration per year. When trying to achieve this behaviour using the library I noticed the parameter passed is only for migration before this date. I couldn't find a way to set a limit for after date.

The behaviour proposed: Being able to run something like:

squasher 2016 2017

or more implicit:

squasher --after 2016 --before 2017