Don't open in a new tab if using web wallet

[davidyuk]

https://github.com/aeternity/superhero-button/blob/4ca4b221c26518233e3b9517173c6b434d9d2100/src/superhero-button.js#L33

Because the web wallet will redirect the user back after tipping and the user doesn't want to have the same page opened twice after that.