feat: allow dashes in begin and end dates

[jrick]

This time format was already allowed for entries in the ledger data file, and it was odd to hit time parse errors for dates written in the same format with the -b/--begin-date and -e/--end-date flags.

[howeyc]

This is nice, but I'm wondering if it would be worthwhile to support many date formats, like in the ledger file parser. Like this:

https://github.com/howeyc/ledger/blob/ffdee7b6994b8b72d9b894df72ba02dce520243b/parse.go#L194

[howeyc]

LGTM