Add mailgun output plugin to riemann.config ns

Add mailgun output plugin to riemann.config ns so it is available within
riemann configuration and explicit `require` statements are not
necessary.
Merge pull request #454 from dlobue/add_mailgun_to_config_ns

Add mailgun output plugin to riemann.config ns
Allow using the incoming webhooks endpoints in the Slack plugin
Merge pull request #458 from AeroNotix/gh-alf-allow-to-use-incoming-webhooks

Allow using incoming webhooks endpoints in the Slack plugin