Added support for external providers as npm modules.

lib/factory.js

@@ -84,8 +84,17 @@ function createClient(options) {
     // Try to load the provider from providers folder
     handler = require('./providers/' + provider);
   } catch (err) {
-    // Fall back to pkgcloud
-    handler = require('pkgcloud').storage;
+    try {
+      // Fall back to using an external provider
+      handler = require(provider);
+      if (!handler || !handler.createClient) {
+        // Fall back to pkgcloud
+        handler = require('pkgcloud').storage;
+      }
+    } catch(err) {
+      // Fall back to pkgcloud
+      handler = require('pkgcloud').storage;
+    }
   }
   patchContainerAndFileClass(provider);
   return handler.createClient(options);
@@ -101,8 +110,13 @@ function getProvider(provider) {
     // Try to load the provider from providers folder
     return require('./providers/' + provider);
   } catch (err) {
-    // Fall back to pkgcloud
-    return require('pkgcloud').providers[provider];
+    try {
+      // Fall back to using an external provider
+      return require(provider);
+    } catch(err) {
+      // Fall back to pkgcloud
+      return require('pkgcloud').providers[provider];
+    }
   }
 }