Feature request: AFTER keyword for addColumn()

Hello, I dont see this in the documentation that is currently live, and searching GitHub for `after` doesn't turn up anything that looks like the feature.

--> it would be nice to specify the `AFTER` keyword when using `addColumn`, which would generate the `AFTER` string in i.e. a `mysql` query.

In cases where we want to create a new column after another existing column, we are having to resort to runSql(), which isn't ideal

thanks!

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