Extracted `pliny-migrate` + multi-env support

Add in a `pliny-migrate` command that has similar behavior to `bundle
exec rake db:migrate` today, but which will live under the Pliny gem so
that we can update it independently.

Also brings in multi-environment behavior so that the values for
`DATABASE_URL` in both `.env` and `.env.test` are respected.

```
$ pliny-migrate
Root is /Users/brandur/Documents/heroku/pliny
Migrating .env
Migrating .env.test
```