Added ability to pass last-modified file as option to spawned proc

Added option to use `fileAsArg: true` to pass file as argument to spawned tasks. This hopefully removes the most common usecase for `nospawn: true` and thus lets people avoid a lot of the bugs that can be encountered when using it. References #148 and #150. 

If you like this sort of implementation, I can probably write tests. However if the maintainers don't like the feature then I wont bother. Let me know if you think you might merge it in.