Introduces the wc_get_log_file_path() function

I tried to change the path to:

```php
define( 'WC_LOG_DIR', dirname( __FILE__ ) . '/wp-content/logs/' );
```

And I saw it wrong:

```
/var/www/wc-dev/wp-content/logs//paypal-04a54e401c5e8ca594510816bb40c689.log
```

But if I use as:

```php
define( 'WC_LOG_DIR', dirname( __FILE__ ) . '/wp-content/logs' );
```

I'll break here:

```
if ( $this->_handles[ $handle ] = @fopen( WC_LOG_DIR . $this->file_name( $handle ) . '.log', 'a' ) ) {
    return true;
}
// will generate: /var/www/wc-dev/wp-content/logspaypal-04a54e401c5e8ca594510816bb40c689.log
```

So I created a function to standardize it and handle with this o/
Also it make simpler for any Third-party plugin to display the log path equal PayPal gateway.