Parse Mode Setting for KeyBoard

[laskarazhar-hiveup]

Usage

$keyboard = Keyboard::create(Keyboard::TYPE_KEYBOARD)->oneTimeKeyboard()->conParseMode('Markdown')
            ->addRow(KeyboardButton::create('☹️')->callbackData('1'))
            ->addRow(KeyboardButton::create('🙁')->callbackData('2'))
            ->addRow(KeyboardButton::create('😐')->callbackData('3'))
            ->addRow(KeyboardButton::create('🙂')->callbackData('4'))
            ->addRow(KeyboardButton::create('😊')->callbackData('5'))
            ->toArray();

[filippotoso]

Please refactor the commit as follows:

1. Make $parseMode null by default
2. Don't add the parseMode parameter if it's null
3. Rename the conParseMode() method into parseMode()

This changes are required for consistency and to maintain the existing behavior.