Add "chromeapp" field to package.json

I'm attempting to make a defacto standard for specifying Chrome App dependency substitutions using the "chromeapp" field in `package.json`.

The "chromeapp" field is just like the "browser" field in `package.json` (see spec here: https://github.com/defunctzombie/package-browser-field-spec) except it's intended for Chrome App targets instead of a generic browser target.

Specifically, since Chrome Apps can use raw sockets, we want to replace 'net' and 'dgram' with 'chrome-net' and 'chrome-dgram', respectively.

package.json

@@ -8,7 +8,11 @@
   },
   "main": "http.js",
   "license": "MIT",
+  "chromeapp": {
+    "net": "chrome-net"
+  },
   "dependencies": {
+    "chrome-net": "^3.3.2",
     "freelist": "^1.0.3",
     "http-parser-js": "^0.4.3"
   },