Add --ignore-scripts option that causes `npm install` to not run preinstall and prepublish scripts

This PR implements https://github.com/isaacs/npm/issues/2817, an `--ignore-scripts` option that causes `npm install` to not run `preinstall` and `prepublish` scripts.

This is useful, e.g., when you want to snapshot source code on one machine and transfer it to another for building, if you are performing JavaScript source analysis that doesn't require building, or if you want to download all of your deps' sources into your IDE for jump-to-definition in the source. More use cases are described at https://github.com/isaacs/npm/issues/2817.

Includes docs and a test.

If there is a better way to make these modifications to `lib/install.js`, let me know and I will update this PR.