Pass additional arguments to fetch.

[mbostock]

The fetch function can take an optional init argument, and it would be nice to have the ability to passthrough an init object to most of the methods in this library (e.g., to set headers on the request, or in the future, to abort fetches).

In the case of d3.tsv / d3.csv, we probably want to allow one or both of an init and row accessor, so we’d need to do a typeof test in the two-argument case to disambiguate between them.