Support BEFORE and AFTER clauses in createColumnConstraint()

## Issue:
Currently when you add a column, it will always be added to the end of the table. However, you often want to group related columns together to ensure that `select *` queries are more readable and that the column order matches the property order in your application data model.

## Solution:
- Add support for MySQL's `BEFORE` and `AFTER` clauses to `createColumnConstraint()` method by adding `before: column` and `after: column` properties to the column `spec`.

---
Want to back this issue? **[Post a bounty on it!](https://www.bountysource.com/issues/61484691-support-before-and-after-clauses-in-createcolumnconstraint?utm_campaign=plugin&utm_content=tracker%2F12292998&utm_medium=issues&utm_source=github)** We accept bounties via [Bountysource](https://www.bountysource.com/?utm_campaign=plugin&utm_content=tracker%2F12292998&utm_medium=issues&utm_source=github).