Fix _parseNQuads: blank node labels may start by [0-9]

Hi,

I had an issue parsing the nquads output from 'rdfstore-js' (results of a construct sparql query).
The issue is that they used blank nodes labels starting with a number (ex: "_:0")

However, from what I understand of http://www.w3.org/TR/n-quads/#grammar-production-BLANK_NODE_LABEL , it seems completely valid.

Mocha tests are still all passing...