Add TypeScript types (#34)

Closes #28.

Author: jbarrus

index.d.ts

@@ -0,0 +1,15 @@
+import { Agent, AgentOptions } from 'agent-base';
+
+declare module ProxyAgent {
+	interface ProxyAgent extends Agent {
+    }
+}
+
+declare const proxy: ProxyAgentConstructor;
+
+interface ProxyAgentConstructor {
+    (options: AgentOptions | string): ProxyAgent.ProxyAgent;
+    new (options: AgentOptions | string): ProxyAgent.ProxyAgent;
+}
+
+export = proxy;