Added Setting constructor taking a std::string.

To address hyperrealm/libconfig#2, a constructor has been added to the
header and source file.

lib/libconfig.h++

@@ -363,6 +363,8 @@ class LIBCONFIGXX_API Config
   public:
 
   Config();
+  Config(std::string const& cfgFile);
+
   virtual ~Config();
 
   void setAutoConvert(bool flag);

lib/libconfigcpp.c++

@@ -312,6 +312,17 @@ Config::Config()
 
 // ---------------------------------------------------------------------------
 
+Config::Config(std::string const& cfgFile)
+  : _defaultFormat(Setting::FormatDefault)
+{
+  _config = new config_t;
+  config_init(_config);
+  config_set_destructor(_config, ConfigDestructor);
+  readFile(cfgFile.c_str());
+}
+
+// ---------------------------------------------------------------------------
+
 Config::~Config()
 {
   config_destroy(_config);