Use account-appropriate commodity symbol, rather than default commodity symbol

[the-solipsist]

I use ISIN as commodity symbols for the securities (shares / mutual funds, etc.) that I hold. It's a pain to copy-paste the commodity symbol, and then add quote marks since they contain mixed alphanumeric characters. It would be far better if hledger-iadd used the commodity symbol that was last used with a particular sub-account / is most frequently used / whichever heuristic you normally use for amounts, etc.

This way, I could just focus on typing amounts, rather than finding a symbol, copy-pasting it, and then typing the amount.

[hpdeifel]

nb: Related to the workflow from #65