Undocumented options

[sammuell]

By inspecting the code, I 'accidentally' stumbled upon the fact that there are more options available than those which are in the documentation, e.g. buttonClass, group, helperElementPadding

https://introjs.com/docs/intro/options https://github.com/usablica/intro.js/blob/38bbe54d5ce3de90287ef309eff246ac5eff89b2/src/index.js#L35

[afshinm]

I will fix the docs today. Thanks @sammuell