Permit indexes to have extra fields.

CPAN.pm allows the index to contain an addtional "comment"
field.  See note here in the source code:

https://github.com/andk/cpanpm/blob/master/lib/CPAN/Index.pm#L376

I would also like to stuff some extra metadata into an index file using
this comment field.  This change allows cpanm to parse such indexes.