Allow explicit config of Slack HTTP connection params
Allow explicit config of Slack HTTP connection params

This PR allows users to configure the HTTP connection to Slack; the
params are passed directly to `clj-http`.

jeanpralo/riemann,abailly/riemann,riemann/riemann,bmhatfield/riemann,eric/riemann,pradeepchhetri/riemann,riemann/riemann,Anvil/riemann,abailly/riemann,bmhatfield/riemann,eric/riemann,aphyr/riemann,jeanpralo/riemann,pradeepchhetri/riemann,Anvil/riemann,jamtur01/riemann,aphyr/riemann,jamtur01/riemann