API Added providerOfUrl method

Added method that allows looking up a given reader to see if it might be able
to handle the passed in URL, and if so, return a ContentReader for that URL
(if possible).

Note that this is possibly a temporary solution to help out with the
[embed] tag issues (see the cdncontent module's CdnEmbedder class), but may
have other use cases - will monitor for this if it becomes useful