Fix content and children props type

[lgraziani2712]

Instead of limiting the elements to one string or one react element, would be better if it allows one (string, number, or react element) or an array of them.

Which, actually, it already works.

[atomiks]

One last thing. Can we wrap the propTypes assignment in a block like this

if (process.env.NODE_ENV !== 'production') {
  Tippy.propTypes = {...}
}

I noticed that CRA doesn't remove it from the production bundle by default

[lgraziani2712]

I was thinking about that but I was going to ask about in another issue heh.

Done!

[atomiks]

Thank you. I think we need to adjust the rollup build file for the UMD version to work properly in a browser environment, it shouldn't be too difficult as I did it for tippy.js itself recently

[atomiks]

Looks like the propType for content is causing problems :s...