Add Element that renders nothing

[sshehata]

Added an element that can be used to render nothing on the failure of some
condition. #90

[fbonetti]

This is pedantic, but I would reword the comment.

Current:

Represents rendering nothing when some condition is not true.

Revised:

Represents rendering nothing. Useful for when you want to optionally render an element.

The current definition makes it sound as if nothing takes a Bool as an argument

[sshehata]

Bad wording on my part. Fixed !