add sharing to weibo.com.
google、facebook and twitter are block by GFW in China，Chinese can
not visit these websites.
add 'weibo' config in option links.sharing,the share link will not be
used by default,users should set the value of 'weibo' to true to turn
on sharing to weibo.com.
Merge pull request #386 from codepiano/china-social-network-sharing

add sharing to weibo.com.