Fixed zero-padding for <episodenum> and <seriesnum>

In --fileprefix or --subdir-format, can now use parameters in the form
<00episodenum> or <00seriesnum> to produce values padded with zeroes.
Padded string width is determined by number of zeroes, so the above
would pad "5" to "05". If also using an optional prefix character,
it must come before padding indicator, e.g., <.00episodenum>.