Step.element should allow more than string like intro.js

[Sascha Nordquist]

The Step type only allows element to be a string

intro.js-react/index.d.ts

Line 9 in 95f06d2

 element?: string; 

intro.js allows:
element?: string | HTMLElement | Element | undefined;

see: https://github.com/DefinitelyTyped/DefinitelyTyped/blob/173aa9174685d78871a5c035b74d856f4aef9cc8/types/intro.js/index.d.ts#L15