generate-migrations now accepts a statement separator

[daninus14]

For multiple database connectors to do migrations with multiple statements, including cl-dbi, the statements must be separated by --;; on a new line. This change provides a way to provide a custom separator when generating migrations.

The default functionality stays the same. This change provides the flexibility to specify a statement separator so that sxql and cl-dbi will work with the generated statements.

Note that to test the functionality, multiple changes have to be done compared to the state of the DB in order to see multiple statements generated.