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/integration/docker/deployer/app/.mrsk/hooks/post-push

@@ -0,0 +1,3 @@
+#!/bin/sh
+echo "Built and pushed!"
+mkdir -p /tmp/${TEST_ID} && touch /tmp/${TEST_ID}/post-push

test/integration/docker/deployer/app/.mrsk/hooks/pre-build

@@ -0,0 +1,3 @@
+#!/bin/sh
+echo "About to build and push..."
+mkdir -p /tmp/${TEST_ID} && touch /tmp/${TEST_ID}/pre-build