feature request : Provide a version control system for fang migrations.

[pxp9]

          Imagine a user is using the current version of fang. and she wants to upgrade to the version with the changes from this pr. she will have to figure out the migrations she has to make herself. 

maybe not in this pr, we can provide a way to migrate fang db automatically

check this out - https://hexdocs.pm/oban/Oban.Migration.html

Originally posted by @ayrat555 in #141 (comment)