Add binding for libpeas & libpeas-gtk

libpeas is the plugin management system by gnome. They use it basically in all their software to allow extending it.

Adding the package to GtkD (because it relies on glib) would allow people to extend other projects using libpeas but also include a default GTK plugin manager & view using libpeas-gtk.

General project docs: https://developer.gnome.org/libpeas/stable/
