Put <content></content> at end of image template

[tdesc]

In earlier versions of element it have.

  <content></content>

It allows to put content over image with very little efforts.
In latest releases it was removed completely.

[bicknellr]

Oh, looks like I did this in #44 without any comment about it.. I don't really think it should be reverted though; iron-image (like img) isn't meant to be a container. I suspect background-* properties might be more useful if that's what you're going for. But, here's a demo of overlapping stuff w/ an iron-image just in case. Do you have a particular use case in mind?

[tdesc]

What i'm doing is really using image as container with background. demo
Having seems to have no additional overhead for iron-image, but gives you power of container. ( as it was in ealier versions )