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`

README.md

@@ -902,6 +902,19 @@ When `limit` is specified, containers will be booted on, at most, `limit` hosts
 
 These settings only apply when booting containers (using `mrsk deploy`, or `mrsk app boot`). For other commands, MRSK continues to run commands in parallel across all hosts.
 
+## Hooks
+
+You can run custom scripts at specific points with hooks.
+
+Hooks should be stored in the .mrsk/hooks folder. Running mrsk init will build that folder and add some sample scripts.
+
+If the script returns a non-zero exit code the command will be aborted.
+
+There are currently two hooks:
+
+- pre-build
+- post-push
+
 ## Stage of development
 
 This is beta software. Commands may still move around. But we're live in production at [37signals](https://37signals.com).