MRSK hooks

Adds hooks to MRSK. Currently just two hooks, pre-build and post-push.

We could break the build and push into two separate commands if we
found the need for post-build and/or pre-push hooks.

Hooks are stored in `.mrsk/hooks`. Running `mrsk init` will now create
that folder and add sample hook scripts.

Hooks returning non-zero exit codes will abort the current command.

Further potential work here:
- We could replace the audit broadcast command with a
post-deploy/post-rollback hook or similar
- Maybe provide pre-command/post-command hooks that run after every
mrsk invocation
- Also look for hooks in `~/.mrsk/hooks`

lib/mrsk/cli/base.rb

@@ -134,5 +134,17 @@ module Mrsk::Cli
           MRSK.hold_lock_on_error = false
         end
       end
+
+      def run_hook(hook)
+        run_locally do
+          if MRSK.hook.hook_exists?(hook)
+            begin
+              MRSK.with_verbosity(:debug) { execute(*MRSK.hook.run(hook)) }
+            rescue SSHKit::Command::Failed
+              raise HookError.new("Hook `#{hook}` failed")
+            end
+          end
+        end
+      end
   end
 end