add esp thingspeak extension (#2068)

docs/extensions.md

@@ -330,6 +330,10 @@ Check out [microbit.org](http://microbit.org/resellers/) for more information on
   "name": "WiFi:Bit",
   "url":"/pkg/e-radionicacom/pxt-wifi",
   "cardType": "package"
+}, {
+  "name": "ESP8266 and ThingSpeak",
+  "url":"/pkg/alankrantas/pxt-ESP8266_ThingSpeak",
+  "cardType": "package"
 }]
 ```
 

targetconfig.json

@@ -129,7 +129,8 @@
             "monkmakes/pxt-sensor",
             "alankrantas/pxt-Nexusbit",
             "elecfreaks/pxt-cutebot",
-            "KitronikLtd/pxt-kitronik-fischertechnik"
+            "KitronikLtd/pxt-kitronik-fischertechnik",
+            "alankrantas/pxt-ESP8266_ThingSpeak"
         ],
         "preferredRepos": [
             "Microsoft/pxt-neopixel",