Add Settings.TestAPI flag to enable using the Telegram test API

[TheoBrigitte]

This PR add the capability to use the Telegram test API in order to help with bot development. The test API setup is describe in the official documentation here https://core.telegram.org/bots/features#creating-a-bot-in-the-test-environment.

After having created a bot in the Telegram test environment you can now create the bot using the new TestAPI setting.

pref := tele.Settings{
	Token:   telegramBotToken,
	TestAPI: true,
}

b, err := tele.NewBot(pref)

I tested this including the file API, I hope I did not missed anything.