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`
Audit details
* Audit logs and broadcasts accept `details` whose values are included as log tags and MRSK_* env vars passed to the broadcast command
* Commands may return execution options to the CLI in their args list
* Introduce `mrsk broadcast` helper for sending audit broadcasts
* Report UTC time, not local time, in audit logs. Standardize on ISO 8601 format
When invoking the audit broadcast command, provide a few environment
variables so that people can customize the format of the message if they
want.
We currently provide `MRSK_PERFORMER`, `MRSK_ROLE`, `MRSK_DESTINATION` and
`MRSK_EVENT`.
Also adds the destination to the default message, which we continue to
send as the first argument as before.
