extra/hledger-register-csv: accept -H/-A/-r (#206)

hledger-register-csv.hs now accepts the same -H, -A and -r flags that
register does (but not -w, which wouldn't make sense).

Note: remember, when running add-ons with the hledger executable, most
add-on command flags must be preceded by --. So it's:

$ hledger register-csv -- -H