Allow for "Bring your own DB"

[bdcorps]

Currently Candymail uses an internal SQLLite DB to keep track of all the emails to be sent out. For more production use cases, we should have the option to specify an external DB.

The config object might look like:

{
    mail: {
      host: 'smtp.gmail.com',
      port: 465,
      secure: true,
      auth: {
        user: process.env.MAIL_USER,
        pass: process.env.MAIL_PASSWORD,
      },
      tls: {
        rejectUnauthorized: true,
      },
    },
    hosting: { url: process.env.HOSTING_URL },
    db: { url: "SQLITE_URL", reset: true },
    debug: { trace: true },
  }