Support for multiverse

[mlohbihler]

We are using the multiverse gem to support multiple databases. I've found that the way that the Config's @dbconfig var is created, Squasher cannot start due to errors trying to create connections to the non-default database. Allowing alternate database configurations fixes the problem. For example, at line 125:

          @dbconfig = {
            'development' => content['development'].merge('database' => 'squasher'),
            'timescale_development' => content['timescale_development'].merge('database' => 'squasher'),
          }

(Changing the timescale database to "squasher" is not actually necessary.)

This is fine for allowing Squasher to squash migrations of the default database, but we also would like to squash those of the timescale database too. My thinking is to add two parameters: one to specify all of the configs that need to be part of @dbconfig. The other is to specify the database that is to be squashed. Does this sound reasonable? If so, i could probably create a PR with a proposed solution.