Added the Valiator method isBoolean

Author: sephedo

README.md

@@ -189,6 +189,20 @@ If the environment variable is not an integer, you'd get an Exception:
 One or more environment variables failed assertions: FOO is not an integer
 ```
 
+### Boolean Variables
+
+You may need to ensure a variable is in the form of a boolean, accepting "On", "1", "Yes", "Off", "0" and "No". You may do the following:
+
+```php
+$dotenv->required('FOO')->isBoolean();
+```
+
+If the environment variable is not a boolean, you'd get an Exception:
+
+```
+One or more environment variables failed assertions: FOO is not a boolean
+```
+
 ### Allowed Values
 
 It is also possible to define a set of values that your environment variable

src/Validator.php

@@ -76,6 +76,21 @@ function ($value) {
             'is not an integer'
         );
     }
+    
+    /**
+     *  Asset that each specified variable is a boolean.
+     *
+     * @return \Dotenv\Validator
+     */
+    public function isBoolean()
+    {
+		return $this->assertCallback(
+			function($value) {
+				return (filter_var($value, FILTER_VALIDATE_BOOLEAN,  FILTER_NULL_ON_FAILURE) !== NULL);
+			},
+			'is not a boolean'
+		);
+	}
 
     /**
      * Assert that each variable is amongst the given choices.