Make enclosure lengths optional.

Not all RSS variants require a length for their enclosures, nor do
producers of RSS2.0 always include a @length attribute for
<enclosure/>. Hence just make the attribute be optional.

That is, change the types of the following two

  Text.RSS.Syntax.RSSEnclosure.rssEnclosureLength
     :: Maybe Integer

  Text.Feed.Query.getItemEnclosure
     :: ItemGetter (String, Maybe String, Maybe Integer)

Downstream compilation breakage for code that works with enclosures is
to be expected..but ought to be trivial to fix.