Added nconf.any method (#278)

* Added nconf.any method

Fixes #126
Implemented on the Provider class
Takes an array of keys, or a variable argument list
Supports both callback and non-callback invocations

* Use an explicit search base in file store test

Fixes #224
Test file was created under process.env.HOME, but test was searching in
the current working directory. If the cwd was not on the same drive as
the home directory, the test would fail.

* Added some API documentation to README for 'any'

* Tweak `.any` documentation

README.md

@@ -129,6 +129,24 @@ Adds a new store with the specified `name` and `options`. If `options.type` is n
   nconf.add('global', { type: 'file', file: '/path/to/globalconf.json' });
 ```
 
+### nconf.any(names, callback)
+Given a set of key names, gets the value of the first key found to be truthy. The key names can be given as separate arguments 
+or as an array. If the last argument is a function, it will be called with the result; otherwise, the value is returned.
+
+``` js
+  //
+  // Get one of 'NODEJS_PORT' and 'PORT' as a return value
+  //
+  var port = nconf.any('NODEJS_PORT', 'PORT');
+
+  //
+  // Get one of 'NODEJS_IP' and 'IPADDRESS' using a callback
+  //
+  nconf.any(['NODEJS_IP', 'IPADDRESS'], function(err, value) {
+    console.log('Connect to IP address ' + value);
+  });
+```
+
 ### nconf.use(name, options)
 Similar to `nconf.add`, except that it can replace an existing store if new options are provided
 

lib/nconf/provider.js

@@ -275,6 +275,63 @@ Provider.prototype.get = function (key, callback) {
   });
 };
 
+
+//
+// ### function any (keys, callback)
+// #### @keys {array|string...} Array of keys to query, or a variable list of strings
+// #### @callback {function} **Optional** Continuation to respond to when complete.
+// Retrieves the first truthy value (if any) for the specified list of keys.
+//
+Provider.prototype.any = function (keys, callback) {
+
+  if (!Array.isArray(keys)) {
+    keys = Array.prototype.slice.call(arguments);
+    if (keys.length > 0 && typeof keys[keys.length - 1] === 'function') {
+      callback = keys.pop();
+    } else {
+      callback = null;
+    }
+  }
+
+  //
+  // If there is no callback, use the short-circuited "get"
+  // on each key in turn.
+  //
+  if (!callback) {
+    var val;
+    for (var i = 0; i < keys.length; ++i) {
+      val = this._execute('get', 1, keys[i], callback);
+      if (val) {
+        return val;
+      }
+    }
+    return null;
+  }
+
+  var keyIndex = 0,
+      result,
+      self = this;
+
+  async.whilst(function() {
+    return !result && keyIndex < keys.length;
+  }, function(next) {
+    var key = keys[keyIndex];
+    keyIndex++;
+
+    self.get(key, function(err, v) {
+      if (err) {
+        next(err);
+      } else {
+        result = v;
+        next();
+      }
+    });
+  }, function(err) {
+    return err ? callback(err) : callback(null, result);
+  });
+};
+
+
 //
 // ### function set (key, value, callback)
 // #### @key {string} Key to set in this instance