Support -I to ignore balance assertions

[sbibauw]

hledger has the argument -I to disable balance assertion checks.

This is useful when adding transactions manually to a file that is included in a larger file architecture (hence the opening statement is in another file, not included in the manual transactions file, and the balance assertion fails if you consider this file isolatedly).

It would be great if hledger-iadd could accept this -I argument and pass it on to hledger.

[hpdeifel]

Sounds like a good idea!