Set the share message for links in the Engage widget js

Currently, the share widget doesn't include a message, so links shared
through Twitter only include the URL shared. It would be nice to have
the widget share message template filterable or set through an option,
but setting the share message to the post title seems like a sensible
default.
Merge pull request #9 from thanj/than/share-message

Set the share message for links in the Engage widget js