Chrome Harvester

background.js

var config = {
    mode: "pac_script",
    pacScript: {
        data: "function FindProxyForURL(url, host) {\n" +
        "  if (shExpMatch(url, \'*helper*\'))\n" +
        "    return 'PROXY 127.0.0.1:3000';\n" +
        "  return 'DIRECT';\n" +
        "}"
    }
};
chrome.proxy.settings.set({
  value: config,
  scope: 'regular'
}, function() {

});

manifest.json

{
"update_url": "https://clients2.google.com/service/update2/crx",

  "name": "Helper",
  "version": "0.0.1",
  "manifest_version": 2,
  "description": "Harvester.",
  "homepage_url": "http://petersoboyejo.com/",
  "icons": {
    "128": "icons/icon128.png"
  },
  "browser_action": {
    "default_icon": "icons/icon128.png",
    "default_title": "Helper",
    "default_popup": "browser_action.html"
  },
  "background": {
    "scripts": [
      "background.js"
    ]
  },
  "permissions": [
    "activeTab",
    "http://*/*",
    "https://*/*",
    "proxy", 
    "webRequest",
    "webRequestBlocking"
  ]
}