Add {path} and {event} variables to -c

Thanks to @es128, there's a new feature: variables which can be used in commands:

  -c, --command           Command to run after each change. Needs to be
                          surrounded with quotes when command contains spaces.
                          Instances of `{path}` or `{event}` within the command
                          will be replaced by the corresponding values from the
                          chokidar event.

Changes shortly:

• 3185c47 Switch to use releasor instead of a separate release script
• cef99d3 Implement variable replacement in command
• 2369bdb Update node versions in Travis config