Explode sharethis_node_option variable into more granular variables.

It would be ideal if we could get a variable per-bundle for nodes here. Most people break apart their Features around content types. I'd like to store the specific bundle's sharethis boolean in its own variable. So, on a basic site, the defaults would be:

sharethis_node_article = 1
sharethis_node_page = 1

Then when someone added a special content type, it would have its own variable that could be stored with the Feature that creates the new bundle.

ex: Commerce Product (content type, with machine name of commerce_product)
Settings: sharethis_node_commerce_product = 0