Add a new option to migrate up/down command to support migration to a specific version

[qhchen88]

This is yet another way to limit the number of migrations. It's easy for users to figure out the target version instead of counting the limit number.

[rubenv]

I like it, but I don't want to break compatibility by adding a new parameter, especially one that should be zero if other parameters are set. It'll cause all existing code using this library to break.

Better to add a new function.

[qhchen88]

Updated:

• Add ExecVersion
• Wrap the apply migrations code into a new function
• Allow the version number to be zero. Some people might choose the start version number as 0.
• Add more test cases

[rubenv]

Ok, this looks great now, purely additive so it doesn't break anything. I see no further reason not to merge this.

Thanks!

[rubenv]

Released as v1.3.0