Fix references to isaacs/npm

Github will forward from [github.com/isaacs/npm](https://github.com/isaacs/npm) to [github.com/npm/npm](https://github.com/npm/npm), but some tools don't know to follow that forward and error out.

I've replaced references to `isaacs/npm` with `npm/npm` in both places where it matters (`package.json`) and places it doesn't really matter but for aesthetic reasons (readme and docs).