Create keeko:migration command(s)

[gossi]

Creates the required migration classes to handle module updates (and most likely) schema updates. Will be using propel migration:diff --skip-removed-table because nobody can remember that option name. Will also need to keep track of migrations and from which version to which version they belong, so a possible composer update command can execute all related migrations. Will also need to work when there is no schema change but a migration class needs to be created.

Most like it will also be necessary to use a keeko publish command (Ref #12).