Local support for 'bugs' and 'docs' commands

Very minor edit to allow the a user to run the `docs`, `home`, `bugs`, `issues` commands without a package argument, in which case they will now attempt to use a local package.json. The attempt at deducing a URL when none is specified in package.json for GitHub projects has also been copied over appropriately to the docs command (previously only implemented in bugs).

Hopefully all OK.

Issue referenced: https://github.com/isaacs/npm/issues/3356