Simplify plugin/hook integration

Previously, if a plugin could be hooked to more than one hook, one
should configure all the integrations by hand. For instance, the
CheckJira plugin can be hooked to the update, the pre-receive, and the
commit-msg hooks, and the user had to know which hooks should be
enabled. Now it's easier because the plugins can hook themselves to
all hooks directly. In this way, for instance, instead of configuring
CheckJira specifically for one of those hooks with one of the
following configurations:

    git config --add githooks.commig-msg  CheckJira
    git config --add githooks.pre-receive CheckJira
    git config --add githooks.update      CheckJira

One can simply do this:

    git config --add githooks.plugin CheckJira

CheckJira will automatically be enabled for all plugins.

Also, the plugins can be referred to by their fully qualified module
names (e.g. Git::Hooks::CheckJira) to allow for third party plugins.

If the githooks.plugin option is not defined, the configuration
processing will try to build it from the old githooks.HOOK
configuration options that are not needed anymore, so to preserve
compatibility. This is a temporary measure.
v0.032

	Simplify plugin/hook integration

	Previously, if a plugin could be hooked to more than one hook,
	one should configure all the integrations by hand. For
	instance, the CheckJira plugin can be hooked to the update,
	the pre-receive, and the commit-msg hooks, and the user had to
	know which hooks should be enabled. Now it's easier because
	the plugins can hook themselves to all hooks directly. In this
	way, for instance, instead of configuring CheckJira
	specifically for one of those hooks with one of the following
	configurations:

	    git config --add githooks.commig-msg  CheckJira
	    git config --add githooks.pre-receive CheckJira
	    git config --add githooks.update      CheckJira

	One can simply do this:

	    git config --add githooks.plugin CheckJira

	CheckJira will automatically be enabled for all plugins.

	Also, the plugins can be referred to by their fully qualified
	module names (e.g. Git::Hooks::CheckJira) to allow for third
	party plugins.

	If the githooks.plugin option is not defined, the
	configuration processing will try to build it from the old
	githooks.HOOK configuration options that are not needed
	anymore, so to preserve compatibility. This is a temporary
	measure.