Ability to determine pages a fixed element shows up on

**Is your feature request related to a problem? Please describe.**
I would like to dynamically render the following layout:
[document.pdf](https://github.com/diegomura/react-pdf/files/3263788/document.pdf)

namely, the ability to dynamically render "page x of y" where x is the current iteration of the fixed element, and y is the total number of rendered fixed elements. I can't hard code these values since the pdf content is generated dynamically.

**Describe the solution you'd like**
I would like to be able to accomplish the above using something similar to Text's `render` prop. Something like:

```jsx
`page ${currentElement} of ${totalElements}`}
```

**Describe alternatives you've considered**
I assumed I would be able to keep track of which pages an element shows up on using the existing `render` prop on text:

```jsx
class SectionText extends React.Component {
appearsOnPage = new Set()

render() {
return {
const appearsOnPagesArray = Array.from(this.appearsOnPages)
const totalElements = appearsOnPagesArray.length
const firstPage = Math.min(...appearsOnPagesArray)
const currentElement = pageNumber - firstPage + 1

return `Page ${currentElement} of ${totalElements}`
}} />
}
}
```

However, it appears `render` is called on Text for _all_ pages, not just the one where Text is actually rendered.