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/main.rb

@@ -132,6 +132,14 @@ class Mrsk::Cli::Main < Mrsk::Cli::Base
       puts "Created .env file"
     end
 
+    unless (hooks_dir = Pathname.new(File.expand_path(".mrsk/hooks"))).exist?
+      hooks_dir.mkpath
+      Pathname.new(File.expand_path("templates/sample_hooks", __dir__)).each_child do |sample_hook|
+        FileUtils.cp sample_hook, hooks_dir
+      end
+      puts "Created sample hooks in .mrsk/hooks"
+    end
+
     if options[:bundle]
       if (binstub = Pathname.new(File.expand_path("bin/mrsk"))).exist?
         puts "Binstub already exists in bin/mrsk (remove first to create a new one)"