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

In --fileprefix or --subdir-format, can now use parameters in the form
<0episodenum> or <0seriesnum> to produce values padded with zeroes.  Add
additional zeroes for additional padding width. If also using an optional
prefix character, prefix must come before padding indicator, e.g.,
<.0episodenum>
Replace trailing ellipsis when sanitising paths

Or Windows will do it for you and cause the download to fail.