Simplify .transform(drop_foreign_keys=) and sqlite-transform --drop-foreign-key

[simonw]

These both currently require you to provide three strings, for column, other_table, other_column.

Just providing column should be enough information.

[simonw]

This is a backwards incompatible change, so technically I should bump the major version to 3. I'm not going to do that, because the feature is brand new and the chance that anyone has written code or shell scripts that use it is vanishingly small.