multi-schema support when parsing/dumping

[peterldowns]

Customers may want to use pgmigrate's parsing/dumping logic with migrations that interact with multiple different Postgres schemas within the same database. Currently, the dumping logic doesn't support this — the code assumes that you're interacting with a single schema, which it assumes by default is named public. This PR changes that assumption and makes it so that you can parse/dump multiple schemas within the same database.

TODO:

• actual details
• config file update
• better testing