Interim Invoice

I think it would be really useful if BrightManager had an interim billing option.

The idea would be that we can create an interim bill and link it directly to a specific task. That bill would then show against the task in the WIP report, so we can see the true WIP position after taking account of amounts already billed.

This would be especially helpful where we bill larger jobs in stages, rather than only at the end. At the moment, the WIP report can make it look like there is more unbilled time sitting on a task than there actually is, because it does not clearly reflect interim bills against that task.