Allow to set collect key from relation definition.

This is needed when declaring a relation hasManyThrough from and to the same classe.

Exemple: 
2 class Profile and Follow

Follow have 2 columns: followingId, followeeId

app.models.profile.hasMany(app.models.profile, {
    through: app.models.follow,
    as: 'followers',
    foreignKey: 'followerId',
    collect: 'follower'
});

app.models.profile.hasMany(app.models.profile, {
    through: app.models.follow,
    as: 'followees',
    foreignKey: 'followeeId',
    collect: 'followee'
});