Watch code and automatically rebuild when something changes

Problem

Compose could watch your code and automatically kick off builds when something changes. This would mean: - Code can be reloaded automatically regardless of language. This would also make code reloading work on compiled languages if that was part of the build process! - Volumes are no longer needed in development - Production server can be used in development (e.g. gunicorn can be used to serve Django apps in development) It could either be an option to `up` (`docker-compose up --watch`), an option in the config file specifying directories to watch/ignore, or perhaps a separate command (`docker-compose watch`). Thanks to @samalba for the suggestion!