Allow balance to show how much is left in a budget

[julian-klode]

I'd like to be able to see how much money is left in a given budget account. Currently you see:

$ hledger -f t bal --budget e
Budget performance in 2019/09/01:

              ||               2019/09/01 
==============++==========================
 <unbudgeted> || -10 EUR                  
 e            ||  10 EUR [ 200% of 5 EUR] 
--------------++--------------------------
              ||       0 [   0% of 5 EUR] 

and you can calculate yourself that you spent 5€ too much.

ledger directly tells me how much money I spent too much (or is left), which can be useful for like knowing how much money you are allowed to still spent on groceries or stuff. But ledger is not really all that readable, without headings and stuff ;):

$ ledger -f t bal --budget e
               5 EUR  e
$ ledger -f t budget e
                10 EUR                  5 EUR                  5 EUR  200%  e

For some t:

$ cat t
~ monthly
        a  -5 EUR
        e  5 EUR

2019/09/01 an expense
        a -10
        e  10 EUR