Create post install script so we can gracefully update internals across versions

[RamonGebben]

When we want to modify something internal from mergify we will have some users that still have the previous version. To not break on update with for instance config changes I suggest we use a post install script to verify that mergify can run, otherwise perform migration.

[jlengrand]

Totally agree.
I'm not sure I understand what postinstall should actually do.

Convert any existing config to the new format?

[RamonGebben]

@jlengrand that was indeed what I was thinking.
However I doubt that it's going to be limited to that.

For instance the issue #29 that you've opened, we could show a message after update that the API has changed and invoke the help message.