[FR] Input relative values

[brunetton]

Hi, first of all thank you for this wonderful app ! This is amazing

Problem

I'm using it since some days to track some expenses, as I can then compare it to other variables, it's absolutely great for me. To get the remaining amount after each expense I've to do a (very simple) operation: total_remaining - expense.
But this is sometimes not convenient to do this calculation, typically when I'm in a hurry, and above all is error prone.

Proposed solution

It would be super nice to be able to write something like @-4.5, that would mean: remove 4.5 to the current value.
(Maybe it should exists a more-or-less "standard" way to input this, the @ is totally random)

Hope I'm clear enough and thank you for your time.