Allow -d to specify per-feed attributes (removed dir support)

Previously the data resolution was for a particular feed value (which maybe
wasn't so clear) but frequently per-feed attributes are needed.  This
necessitated the addition of a nesting key to the data file because JSON will
not allow numeric keys.

There isn't a good generic solution for how to add this data for a directory,
without specifying another directory specifically for the feed attributes; all
reasonable file names (ie those without spaces) are accessible from the feed
values, so there is no room for per-feed files.  This being the case the
directory-style has been removed.  Maybe this is a pure good as the directory
style is hard to explain alongside the file solution.
Allow data_index in data file to be an object

Having the indexes makes it somewhat easier to reason about in some cases.
Added as a convenience (which might end up being the preferred way, who knows).
Warn data file misconfiguration for missing data_index

This happens easily if you pick 1 instead of 0 offset.  Not as easy to detect
if you don't add enough data... maybe expand this later.