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/build.rb

@@ -16,6 +16,7 @@ class Mrsk::Cli::Build < Mrsk::Cli::Base
 
       verify_local_dependencies
 
+      run_hook("pre-build")
       run_locally do
         begin
           MRSK.with_verbosity(:debug) { execute *MRSK.builder.push }
@@ -31,6 +32,7 @@ class Mrsk::Cli::Build < Mrsk::Cli::Base
           end
         end
       end
+      run_hook("post-push")
     end
   end