Add TypeScript type definitions (#66)

* Added index.d.ts

Based on: #27 (comment)
(thank you @yonilerner)

* Update package.json

index.d.ts

@@ -0,0 +1,22 @@
+declare module 'https-proxy-agent' {
+    import * as https from 'https'
+
+    namespace HttpsProxyAgent {
+        interface HttpsProxyAgentOptions {
+            host: string
+            port: number
+            secureProxy?: boolean
+            headers?: {
+                [key: string]: string
+            }
+            [key: string]: any
+        }
+    }
+
+    // HttpsProxyAgent doesnt *actually* extend https.Agent, but for my purposes I want it to pretend that it does
+    class HttpsProxyAgent extends https.Agent {
+        constructor(opts: HttpsProxyAgent.HttpsProxyAgentOptions)
+    }
+
+    export = HttpsProxyAgent
+}

package.json

@@ -3,6 +3,7 @@
   "version": "2.2.1",
   "description": "An HTTP(s) proxy `http.Agent` implementation for HTTPS",
   "main": "./index.js",
+  "types": "./index.d.ts",
   "scripts": {
     "test": "mocha --reporter spec"
   },