Add support for multiple redis connections

[SxDx]

This PR adds support for more than one redis connection by providing multiple configurations to redis_options like this:

Modis.redis_options = {
  default: { url: 'redis://localhost:6379/0' },
  custom: { url: 'redis://localhost:6379/1' }
}

while still being backwards compatible to the old configuration style

Modis.redis_options = {
 url: 'redis://localhost:6379/0
}

You can then select which connection to use on a per model basis

class User
  include Modis::Model
  self.modis_connection = :custom

  attribute :name, :string
end

If you have any ideas on how to improve the tests I am more than happy to change them.

[SxDx]

Hey @ileitch and @aried3r 👋 ,
We would really love this feature for our app.
Is there a chance that you could take a look at this, or should we look at forking the project?