Feature: Read matchers from config file

[mjr9804]

Adds support for loading a custom config file that contains the matchers to be used in spotting certain NPM errors and triggering retries.

The goal is to make it simple for users to customize which NPM errors they need to match on without changing functionality of existing syntax.

By default, the same current list of supported matchers is loaded from .npmretry.json:

{
  "matchers": ["npm ERR\\! cb\\(\\) never called\\!", "npm ERR\\! errno ECONNRESET",
               "npm ERR\\! shasum check failed", "npm ERR\\! code EINTEGRITY"]
}

Users can also create a ~/.npmretry.json or specify a path to a file with --configFile