Fix Open-graph meta data image and url
Fix Open-graph meta data image and url

Issue #68. Title and description don't work because the url of the metadata is wrongly configured. Change from site.url -> site.url/page.url

Another small improvement is that when the user uses a featured image in their post, it should be displayed in the sharing link instead of the icon of the site.
renyuanz/leonids,thechargedneutron/thechargedneutron.github.io,thechargedneutron/thechargedneutron.github.io,xiaobing211314/xiaobing211314.github.io,renyuanz/leonids,xiaobing211314/xiaobing211314.github.io,thechargedneutron/thechargedneutron.github.io,lavelee/blog,xiaobing211314/xiaobing211314.github.io