Allow removal of values from a cell formula

[psgg1981]

Allow removal of values from a cell formula in order to allow user to quickly correct a mistyped expense.

Example: cell contains =1+2+3 and 2 is to be removed; final result would be =1+3

This should also work with single value cells, of course - e.g. cell with value 3 should become empty if such expense is removed.

Whenever the value to be removed belongs to a product operation - e.g. value 3 in =1+2+3*2 - its multiples should be decremented i.e. =1+2+3*1

[psgg1981]

In order for this feature to be successfully executed (and not confused with #8) it must use a new argument

e.g. add_expense --remove 3 99.99 should make formula =1+2+3+4+5+6+7+99.99 into =1+2+3+4+5+6+7

Also, for the sake of numeric comparison, all numbers should be properly compared to currency (2 decimal places)

e.g. add_expense --remove 1 3.00 should still make formula =1+2+3+4+5+6+7 into =1+2+4+5+6+7