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/commands/hook_test.rb

@@ -0,0 +1,32 @@
+require "test_helper"
+
+class CommandsHookTest < ActiveSupport::TestCase
+  include ActiveSupport::Testing::TimeHelpers
+
+  setup do
+    freeze_time
+
+    @config = {
+      service: "app", image: "dhh/app", registry: { "username" => "dhh", "password" => "secret" }, servers: [ "1.1.1.1" ],
+      traefik: { "args" => { "accesslog.format" => "json", "metrics.prometheus.buckets" => "0.1,0.3,1.2,5.0" } }
+    }
+
+    @performer = `whoami`.strip
+    @recorded_at = Time.now.utc.iso8601
+  end
+
+  test "run" do
+    assert_equal [
+      ".mrsk/hooks/foo",
+      { env: {
+        "MRSK_RECORDED_AT" => @recorded_at,
+        "MRSK_PERFORMER" => @performer,
+        "MRSK_VERSION" => "123" } }
+    ], new_command.run("foo")
+  end
+
+  private
+    def new_command
+      Mrsk::Commands::Hook.new(Mrsk::Configuration.new(@config, version: "123"))
+    end
+end