Project-specific releaser hooks in setup.cfg

This pull request adds a feature I've been wanting a long time which is the ability to list project-specific releaser hooks in the setup.cfg for a project under the `[zest.releaser]` section.

Each hook point (prereleaser.before, etc) is given as one or more fully qualified Python function names.  Just as with the entry_points, each hook function takes the data dict as its sole argument.  entry_points are still run, but only after the project-specific hooks.  This eliminates the annoyance of having to install my project just so that the correct entry_points are installed in order to do a release.

No tests yet, but if this approach looks good I'll add some.  FWIW I tested this with one of my own projects and it works as expected.