Support for more number formats by removing non-number characters

The CSV files from my (German) bank contain commas as decimal separator, dots "." as thousands separator and plus signs "+" for positive amounts. E.g.:

    +160,00
    -1.000,00

The plus signs and the thousands separators are not handled by ledger and should be removed from the output.

Instead of trying to support more number formats, I propose to simply strip off any offending characters which are not digits 0-9, minus "-" or the decimal separator "." or ",".