Allow config defaults to be sourced from ENV

This allows the default config to source values from ENV variables if
they are set. With this change we don't have to modify the docker CMD or
edit the config.yml and then relaunch.

src/util/util.cr

@@ -95,9 +95,9 @@ class String
   end
 end
 
-def env_is_true?(key : String) : Bool
+def env_is_true?(key : String, default : Bool = false) : Bool
   val = ENV[key.upcase]? || ENV[key.downcase]?
-  return false unless val
+  return default unless val
   val.downcase.in? "1", "true"
 end