Simplified custom migration templates

I was adding a custom migration template to a project the other day and I noticed that because the template gets included, obviously it also gets parsed, so I ended up with this monstrosity of a template to get things working: 

```php
<?php
echo <<<PHP
<?php
use Phpmig\Migration\Migration;

class $className extends Migration
{
        /**
         * Do the migration.
         */
        public function up()
        {
            \$container = \$this->getContainer();
        }

        /**
         * Undo the migration.
         */
        public function down()
        {
            \$container = \$this->getContainer();
        }
}

PHP;
?>
```

I've submitted a pull request that changes the way the custom template is imported to use `file_get_contents()` and `sprintf` so that the template becomes much simpler: 

```php
<?php
use Phpmig\Migration\Migration;

class %s extends Migration
{
        /**
        * Do the migration.
        */
        public function up()
        {
            $container = $this->getContainer();
        }

        /**
         * Undo the migration.
        */
        public function down()
        {
            $container = $this->getContainer();
        }
}

```

I've also added a bit of documentation to explain how to use a custom migration template.

I'd appreciate any comments on this change. Thanks.