Add handleInitialRequest option to support #301

The custom filtering logic is not part of the public repository, to
keep the project clean.

lib/cors-anywhere.js

@@ -256,6 +256,7 @@ function parseURL(req_url) {
 // Request handler factory
 function getHandler(options, proxy) {
   var corsAnywhere = {
+    handleInitialRequest: null,     // Function that may handle the request instead, by returning a truthy value.
     getProxyForUrl: getProxyForUrl, // Function that specifies the proxy to use
     maxRedirects: 5,                // Maximum number of redirects to be followed.
     originBlacklist: [],            // Requests from these origins will be blocked.
@@ -310,6 +311,10 @@ function getHandler(options, proxy) {
 
     var location = parseURL(req.url.slice(1));
 
+    if (corsAnywhere.handleInitialRequest && corsAnywhere.handleInitialRequest(req, res, location)) {
+      return;
+    }
+
     if (!location) {
       // Invalid API call. Show how to correctly use the API
       showUsage(corsAnywhere.helpFile, cors_headers, res);