Add idleTimeout and placeholder props

[shekharkhedekar]

Adds the following props for increased customization:

• idleTimeout Allows users to customize the delay timeout for window.requestIdleCallback
• placeholder Allows users to pass a JSX element as the placeholder for increased flexibility over placeholderElement/placeholderClass

[shekharkhedekar]

@GusRuss89 @daleperforce this has been useful in our project - we ended up using a quicker timeout (200) and a react component as the placeholder. LMK if this is something you'd be open to.

[daleperforce]

@shekharkhedekar Please explain why you need a new placeholder instead of using the placeholderElement?

The timeout as a configurable seem ok to me.

[shekharkhedekar]

@shekharkhedekar Please explain why you need a new placeholder instead of using the placeholderElement?

The timeout as a configurable seem ok to me.

@daleperforce Good question!

We have a loading skeleton react component that we would like to put in, so our usage ends up looking like this:

<RenderIfVisible
    visibleOffset={2000}
    idleTimeout={200}
    stayRendered
    placeholder={
        <div className="container">
            <LoadingSkeleton />
        </div>
    }
>
    <MyRepeatingElement />
</RenderIfVisible>;