ShareDaddy: Add `get_share_title` function & filter it.

Create a global `get_share_title()` function in the parent class, with a filter `sharing_title` to allow overriding the title via code.
Props @adamtow from http://recode.net/
ShareDaddy: add filter sharing_display_link.
Allows us to completely override the link for the share buttons
Necessary when you want to create sharing links on something that is not a post
Props @gibrown