Add attribute `@path` to Convertible module

This is an alternative way to make sure `self.path` will exist, without overriding any [functions named path only available after the YAML data has been parsed](https://github.com/mojombo/jekyll/blob/c78c4159c0657d0eda317340721494278318b6bb/lib/jekyll/page.rb#L121).

Alternative to https://github.com/mojombo/jekyll/pull/1587