Allow specifying both start and end date for transaction downloads

[501st-alpha1]

Currently the only way to change the time period of downloaded transactions is to use the --download-days argument, which specifies how many days before today to start downloading transactions. I would like to be able to specify both a start and an end date for the transactions to download. E.g.: I might want to download my transactions for the previous month, but didn't get around to it until the 5th, so I need a way to exclude the transactions that occurred in the past 5 days.

I have a solution that I hacked together that "works", but I'd like some advice before I turn it into a PR. Would you prefer that I extend the current functionality, adding an argument to specify how many days to exclude? Or should I add new arguments that allow the user to specify a start and an end date instead? For a scenario like I described above, the latter might be better, so that I wouldn't have to calculate how many days I need to use for each argument. Alternatively, should we support both types of arguments?