Update default processor function signature.

This has been updated to match the other processors handler function, recieving a resolve and reject
function as the 2nd and 3rd arguments respectively. This fixes closes #1881 with gists not rendering
correctly.
Ensure *all* rendering goes through processors

Allows for consistent debugging, and longer life for hair in head.
Merge pull request #1886 from jsbin/fix/1881

Update default processor function signature.