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/templates/sample_hooks/post-push.sample

@@ -0,0 +1,20 @@
+#!/bin/sh
+
+# A sample post-push hook
+#
+# Checks:
+# 1. We have a clean checkout
+#
+# These environment variables are available:
+# MRSK_RECORDED_AT
+# MRSK_PERFORMER
+# MRSK_VERSION
+# MRSK_DESTINATION (if set)
+
+if [ -n "$(git status --porcelain)" ]; then
+  echo "Git checkout is not clean, aborting..."
+  git status --porcelain >&2
+  exit 1
+fi
+
+exit 0