force=TRUE option for loop_tasks

Splitting this issue off from #4

It would be swell if loop_tasks, or scmake called on a recipe with command=loop_tasks, could accept a force=TRUE argument in which case it would delete some/all tasks and/or steps before attempting to rebuild them.

Maybe even while setting aside the deleted files during the build so they can be replaced if the build encounters errors (but that's optional icing on the cake).

And it would be triply swell if that command also gave us the option to choose which tasks and steps to force the rebuild for, and which to leave alone. Maybe an additional argument that looks something like force_steps=c(1,4,5) or force_tasks=c(3,6)?