Configure default key options

[LaravelFreelancerNL]

ArangoDB supports several key generators. The default is 'traditional', somewhat akin to the MySQL auto increment generator.

If you want to use a different one throughout your project you now have to edit each migration file.

Making the key options configurable lets a user set it for all migration files. Some commands like autoincrement and uuid on either the 'id' or '_key' field should still override the default.