Add basic CI setup (#30)

Co-authored-by: Rafael Epplée <hello@rafa.ee>
Reviewed-on: #30
Co-authored-by: Moritz <moritz.m@local-it.org>
Co-committed-by: Moritz <moritz.m@local-it.org>

mix.exs

@@ -57,7 +57,10 @@ defmodule Mv.MixProject do
       {:gettext, "~> 0.26"},
       {:jason, "~> 1.2"},
       {:dns_cluster, "~> 0.1.1"},
-      {:bandit, "~> 1.5"}
+      {:bandit, "~> 1.5"},
+      {:mix_audit, "~> 2.1", only: [:dev, :test], runtime: false},
+      {:sobelow, "~> 0.13", only: [:dev, :test], runtime: false},
+      {:credo, "~> 1.7", only: [:dev, :test], runtime: false}
     ]
   end