Add Greeter functionality

[eruizdechavez]

I believe someone mentioned that we could use the bot also to welcome new members that join the community. This can be done very easily by adding an event handler for workspace join, or simply by adding a channel join listener for #general (all members must be at the very least in this channel.

Some highlights that can be useful:

• Message can be setup in a new options table in the DB.
• This could also be helpful to add special greetings for special channels, like #bolsadetrabajo, #ayuda-tecnica, #meetup, etc.

[eruizdechavez]

With the events API we can monitor team_join event and send a direct message.

https://api.slack.com/events/team_join

For custom channel messages we can just use the regular chanel_join event and a custom message for that channel.

The messages should be on a greeter table, and the initial (default) messages should be part of a migration.

[eruizdechavez]

This is a parent of #16.