Add option for two-part configs with the destination option

lib/mrsk/cli.rb

@@ -1,9 +1,9 @@
 require "mrsk"
 
-MRSK = Mrsk::Commander.new \
-  config_file: Pathname.new(File.expand_path("config/deploy.yml"))
-
 module Mrsk::Cli
 end
 
+# SSHKit uses instance eval, so we need a global const for ergonomics
+MRSK = Mrsk::Commander.new
+
 require "mrsk/cli/main"