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/commander.rb

@@ -100,6 +100,14 @@ class Mrsk::Commander
     @healthcheck ||= Mrsk::Commands::Healthcheck.new(config)
   end
 
+  def hook
+    @hook ||= Mrsk::Commands::Hook.new(config)
+  end
+
+  def lock
+    @lock ||= Mrsk::Commands::Lock.new(config)
+  end
+
   def prune
     @prune ||= Mrsk::Commands::Prune.new(config)
   end
@@ -112,10 +120,6 @@ class Mrsk::Commander
     @traefik ||= Mrsk::Commands::Traefik.new(config)
   end
 
-  def lock
-    @lock ||= Mrsk::Commands::Lock.new(config)
-  end
-
   def with_verbosity(level)
     old_level = self.verbosity