Add period / fd / td date-range overrides to download()

IBKR's v3 Flex Web Service can resolve a query's saved "Last N Calendar
Days" period to a single day when SendRequest carries only v/t/q,
silently returning a statement with no cash transactions or trades. The
documented `p` (period) and `fd`/`td` (from/to date) SendRequest
overrides address this, but the client had no way to pass them.

Add optional `period` (`p`, lookback in days, max 365) and `fd`/`td`
(`yyyymmdd`) kwargs to download() / request_statement / submit_request,
applied only to the SendRequest step (not the GetStatement polls).
`period` and `fd`/`td` are mutually exclusive, and `fd`/`td` must be given
together; invalid combinations raise ValueError, surfaced as a clean
argparse error in the flexget CLI. Matching --period/--fromdate/--todate
flags added to flexget. Default behavior is unchanged.

Addresses #109.