Let blocking be turned-off on a per-file basis using local variables

Let blocking be turned-off on a per-file basis using local variables

Wes Hardaker writes:

> Attached is a patch that lets local variables define whether or not todo
> dependency blocking should be used (both for TODOs and for checkboxes).
> I have one file in particular that I'm using checkboxes to quickly
> indicate multi-selections from a list but for most of my files I want
> TODOs blocked by uncompleted checkboxes.
>
> Normally org uses hook methods for checking for TODO blocks and this
> patch just inserts a check at the top to test and see if the variable
> turning on the blocking type is still set.