Load a collection of Ragtime migrations from a directory.

Load a collection of Ragtime migrations from a classpath prefix.

Given a db-spec and a map of options, return a Migratable database.
The following options are allowed:

:migrations-table - the name of the table to store the applied migrations
                    (defaults to ragtime_migrations)

:migrations-table-exists-sql - SQL string to check if migrations table
                               exists. If not supplied, database metadata is
                               used instead, which can be slow for large DBs.

Create a Ragtime migration from a map with a unique :id, and :up and :down
keys that map to ordered collection of SQL strings.