Add a pre-connect hook

This can be used for hooks that should run before connecting to remote
hosts. An example use case is pre-warming DNS.

lib/mrsk/cli/templates/sample_hooks/post-deploy.sample

@@ -12,6 +12,8 @@
 # MRSK_RECORDED_AT
 # MRSK_PERFORMER
 # MRSK_VERSION
+# MRSK_HOSTS
+# MRSK_ROLE (if set)
 # MRSK_DESTINATION (if set)
 # MRSK_RUNTIME