Fix type definitions to account for transparent proxy discovery (#57)

ProxyAgent constructor can be invoked without a parameter.
In that case, the proxy uri is retrieved from environment variables.

Relevant:
- https://github.com/TooTallNate/node-proxy-agent/blob/master/test/test.js#L166-L214
- #56

index.d.ts

@@ -8,8 +8,8 @@ declare module ProxyAgent {
 declare const proxy: ProxyAgentConstructor;
 
 interface ProxyAgentConstructor {
-    (options: AgentOptions | string): ProxyAgent.ProxyAgent;
-    new (options: AgentOptions | string): ProxyAgent.ProxyAgent;
+    (options?: AgentOptions | string): ProxyAgent.ProxyAgent;
+    new (options?: AgentOptions | string): ProxyAgent.ProxyAgent;
 }
 
-export = proxy;
+export = proxy;