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`

test/cli/cli_test_case.rb

@@ -14,4 +14,17 @@ class CliTestCase < ActiveSupport::TestCase
     ENV.delete("MYSQL_ROOT_PASSWORD")
     ENV.delete("VERSION")
   end
+
+  private
+    def fail_hook(hook)
+      @executions = []
+      Mrsk::Commands::Hook.any_instance.stubs(:hook_exists?).returns(true)
+
+      SSHKit::Backend::Abstract.any_instance.stubs(:execute)
+        .with { |*args| @executions << args; args != [".mrsk/hooks/#{hook}"] }
+      SSHKit::Backend::Abstract.any_instance.stubs(:execute)
+        .with { |*args| args.first == ".mrsk/hooks/#{hook}" }
+        .raises(SSHKit::Command::Failed.new("failed"))
+    end
+
 end