Ignore `through` keyword when validating collections

Many-to-Many Through associations are currently defined with the `through` keyword in

    "attributeName" : {
      collection : "..",
      through: ".."
    }

This used to work great in sails 10-rc5, but throws "Unkown rule: through" in 10-rc7, so I guess it should be added here to prevent that error.
Or did the internal syntax for defining Many-to-Many Through change?

Cheers!
M
Merge pull request #444 from mphasize/patch-1

Ignore `through` keyword when validating collections