Add a configuration method that allows binding non-public properties

[MrDave1999]

Example:

var settings = new EnvBinder()
     .AllowBindNonPublicProperties() // configuration method
     .Bind<AppSettings>();

The AllowBindNonPublicProperties method will tell the binder to bind the non-public properties. By default, the binder only binds the public properties.

This issue is related to #73.