Allow dumping data instead of just schema

[stavros-k]

Currently dbmate adds --schema-only on DumpSchema. Which is expected.

But there are cases where inserts are "part" of the schema/migrations.

ie default settings of the app, a table with predefined set of rows that is used as a "rich enum" (FK from other tables) etc.

These would be good to be included in the schema dump.
I personally dump the schema on a clean database, right after migrations, so git diffs show what actually changed.

So my request is to be able to dump without the --schema-only flag

[ppechkurov]

We have exactly this case: there are templates table and we add some predefined templates via INSERT INTO templates statements. However, they are not present in a dump from dbmate, and this is the issue.