Add support for setting slack application in deploy

[RyanMacG]

Allow for users to set :slack_application in their deploy files without changing default behaviour if they don't.

[parkr]

Hey @RyanMacG! Thanks for the pull request. :) What's your motivation for this? Thanks!

[RyanMacG]

I'm a previous user of capistrano-slack and enjoyed having the functionality there. Also at work we use the application's URL for :application which means all of our deploys have odd links in them as a result.

I get that it might be a niche feature but it's one I'm fond of

[parkr]

Ok, no problem.

There's another line that's using fetch(:application) – can you update that as well? We should probably use a helper method like #slack_application.

[parkr]

What do you think about slack_app_name instead of slack_application?

[RyanMacG]

That sounds good to me, I'll make these changes and push them up

[parkr]

Should this be consistent with the variable name, i.e. slack_app_name?

[parkr]

Thanks mate!

[parkr]

Released in 1.1.0