Support Postgres as a mirroring DB

[princed]

I realize this is a bit out of the blue, but thanks to the use of Sequelize it turned out to be quite easy to enable mirroring to Postges as well.

There's obviously more to it, like dialectOptions (see below) added to each mirror model, but I think it could be useful for someone else even if not accepted.

dialectOptions: {
  charset: 'utf8mb4',
  dateStrings: true,
  typeCast: true,
},

Haven't touched the docs yet, as I'm keen to know if this is something of interest at all.

[TheLastCicada]

Thanks for this pull request @princed . Not sure if this is a priority on this project, or it is desired to support Postgres, but this is on our list and we'll review it!