Migrations support for lightwood

[ea-rus]

It would be great to be able to use models created on old version of lightwood with new version. It can be made by:

• using migrations. Model files and configs should be updated from version to version with migration scripts
• using extendable lightwood API. Runed model should call api functions from exposed lighwood api. New version of lighwood should keep method signature back compatible (by adding a new optional parameter to existing method or create new method.)

To be able to do this we might need to get rid off generation of code for model and replace it with configuration files