addon: Pass module object to NODE_MODULE init function

mainly to allow native addons to export single functions on
rather than being restricted to operating on an existing
object.

Init functions now receive exports as the first argument, like
before, but also the module object as the second argument, if they
support it.

Related to #4634

cc: @rvagg
doc: NODE_MODULE() to pass full `module` to addons

mainly to allow native addons to export single functions on `exports`
rather than being restricted to operating on an existing `exports`
object.

added link to addons repo in docs
doc: fix line wrapping in addons.markdown