Add basic support for DEFAULT values

This commit adds basic support to set `DEFAULT` collumn attributes.
I tested this only with postgres. While gorm cannot use it properly (as the structs are filled with data by default), it can at least create the tables in the database correctly.

This maybe a starting point to add proper `DEFAULT` support to gorm.
Yes, there are a lot of typos in the original code (typ instad of type, etc). I adapted it the way it is to not mess with the other code.