Make email/slack strategy and their dependencies optional.

[LostKobrakai]

This is a breaking change, because dependencies for the email/slack strategy will no longer be installed with Ravenx, but each user does need to install those dependencies manually and add the strategy to his config.

On the flip side ravenx does will not install any unnecessary dependencies a user might not need. This will (at least partially) solve #28.

[odarriba]

Hi @LostKobrakai

I'm sorry about the delay on this.

I cannot accept your merge because it will really generate issues to existing users, causing errors and potentially production issues when they don't realise that some dependencies are missing and the code try to find a module that is not there.

I know that that's the responsibility of each one, but I know that this will cause troubles out there.

But! There is a plan to separate the different strategies into other modules, and that's the way we are going to follow for this, preparing the next major version removing those dependencies, and also the code that needs them, into different packages.

Even not being able to merge this PR, thank you for your effort, every piece of community effort is valuable! ❤️