New squash command

[luchiniatwork]

The idea would be something like this:

$ lein migrana squash ts1->ts2

This would squash all migrations from ts1 to ts2 onto ts2 (effectively removing all intermediary states).

The why: I'm thinking about two scenarios:

1. you are coding and applying schema changes one after another and end up with a bunch of inferred migrations. You could squash them all into one before committing.
2. a project is a bit older and you want to trim down the size of the resources/migrations folder

Potential caveat: squashing would have to be limited to non-tx-fn migrations?