Allowing Passing Commodities to Fetch OR Commodites to Exclude?

[prikhi]

From @simonmichael:

I wondered how it would be to handle args like:

hledger-stockquotes [ COMM ... | not:COMM ... ]

where no args means all the commodities in the journal (except AUTO, for now); negative args (beginning with not:) excludes those commodities; positive args selects only those commodities; and a mix of positives and negatives is an error.