Add support for share post plugin

Fix #24
Change double quotes to single quotes
Move COMMENTS_INTRO to _defaults file
Change style of Comments intro and share post intro

Fix #31
Make hr a bit thicker to make it prominent. 1px is too light to be noticeable
Add line at the end of file
Change check for article.modified which is no longer needed after change in Pelican (>3.3) code

See these discussions

https://github.com/getpelican/pelican/pull/1161
https://github.com/getpelican/pelican/pull/1148
Add TWITTER_USERNAME and GOOGLE_PLUS_PROFILE_URL in _defaults

It will be used for social SEO.

Update #15
Add Google Authorship code to article and page

Update #15
Add social SEO and Google Authorship code to index

Update #15
Add SEO metadata to article and page

Update #15