Problem with migrating down

[mkovel]

Hi, @wesleytodd
I'm trying to create a custom SQL store for migrations, and I want to store migrations log not as one json stringified row.
I want organized it as one row - one migration.

All works fine for up migrations but for down migrations in save method I have problems.
I can't understand the context (up or down) of invoking save method.
And I see two reasons.

1. save does not have access to direction value.
2. set.lastRun has a specific value, that does not reflect really last running migration for down. lastRun has incorrect value when migrating down #142
   But I understand that this trick with lastRun allows work whole system in general.

So I see two ways how it can be fixed.

1. add lastDirection prop to set.
   

2. Change/extend signature of store save method. Add third argument context. save(set, fn, ctx);
   And pass the context with { direction } to store.save.
   
   

I can prepare PR if you agree with my suggestion and select one of them.

[mkovel]

@wesleytodd @tj Guys what do you think?