Pre-release checks should be plugins

The current version, before proceeding, looks for files containing a string specific to holdenweb development practices. Other users will clearly wish to handle their own situations.

The best solution would be to build a plugin system with a simple enough API that each path in the distribution can be examined by each installed plugin. Given a file, plugins are expected to return a go/no-go result for each file. A null plugin would simply return `True`.

Plugins are identified by the `release_` prefix on their names, and must implement a `check(file_path)` function that normally returns `None`, but if the release should not proceed returns a message containing the reason for rejection.