add proxy

PR-URL: #1859
Credit: @badeggg
Close: #1859
Reviewed-by: @isaacs

EDIT: changed flatOptions.noproxy to flatOptions.noProxy, since
make-fetch-happen reads it from the camel-cased property.

lib/utils/flat-options.js

@@ -209,7 +209,13 @@ const flatOptions = npm => npm.flatOptions || Object.freeze({
   ...getScopesAndAuths(npm),
 
   // npm fund exclusive option to select an item from a funding list
-  which: npm.config.get('which')
+  which: npm.config.get('which'),
+
+  // socks proxy can be configured in https-proxy or proxy field
+  // note that the various (HTTPS_|HTTP_|)PROXY environs will be
+  // respected if this is not set.
+  proxy: npm.config.get('https-proxy') || npm.config.get('proxy'),
+  noProxy: npm.config.get('noproxy')
 })
 
 const getPreferOnline = npm => {

tap-snapshots/test-lib-utils-flat-options.js-TAP.test.js

@@ -66,6 +66,7 @@ Object {
   "metricsRegistry": "metrics-registry",
   "nodeBin": "/path/to/some/node",
   "nodeVersion": "1.2.3",
+  "noProxy": "noproxy",
   "npmBin": "/path/to/npm/bin.js",
   "npmCommand": null,
   "npmSession": "12345",
@@ -83,6 +84,7 @@ Object {
   "prefix": "/path/to/npm/cli",
   "preid": "preid",
   "projectScope": "@npmcli",
+  "proxy": "proxy",
   "readOnly": "read-only",
   "rebuildBundle": "rebuild-bundle",
   "registry": "registry",

test/lib/utils/flat-options.js

@@ -110,6 +110,8 @@ class MockConfig {
       'user-agent': 'user-agent',
       '@scope:registry': '@scope:registry',
       '//nerf.dart:_authToken': '//nerf.dart:_authToken',
+      'proxy': 'proxy',
+      'noproxy': 'noproxy',
       ...opts
     }]
   }