pre- and post-snapshot hooks

* stack-based execution model, documented in documentation
* circbuf for capturing hook output
* built-in hooks for postgres and mysql
* refactor docs, too much info on the jobs page, too difficult
  to discover snapshotting & hooks

Co-authored-by: Ross Williams <ross@ross-williams.net>
Co-authored-by: Christian Schwarz <me@cschwarz.com>

fixes #74

logger/nulllogger.go

@@ -13,6 +13,7 @@ func (n nullLogger) ReplaceField(field string, val interface{}) Logger { return
 func (n nullLogger) WithField(field string, val interface{}) Logger    { return n }
 func (n nullLogger) WithFields(fields Fields) Logger                   { return n }
 func (n nullLogger) WithError(err error) Logger                        { return n }
+func (nullLogger) Log(level Level, msg string)                         {}
 func (nullLogger) Debug(msg string)                                    {}
 func (nullLogger) Info(msg string)                                     {}
 func (nullLogger) Warn(msg string)                                     {}