Move plugin config options to a githooks subsection

Previously, each plugin defined a new configuration section for its
own config options. This polluted unnecessarily the configuration
namespace. Now each plugin uses a subsection of the C<githooks>
configuration section. For example, instead of C<checklog.spelling> we
use now C<githooks.checklog.spelling>.

Also, error messages are now prefixed by the fully qualified plugin
name, and not just by the plugin's basename.
v0.031

        Move plugin config options to a githooks subsection. For
        example, the CheckLog.spelling option now is called
        githooks.checklog.spelling. The new scheme avoids unecessary
        pollution of the configuration namespace. The plugins that
        existed up to the previous release can still have their
        options in the previous place, because we dynamically move
        them. This is a temporary measure to avoid breaking old
        configuration. It will probably be removed in the future.

        Rename TODO to TODO.pod and update it with new ideas.