0.7.0

Features:

• placeholder is now optional
  This is motivated by the fact that more advanced placeholder strategies do not even put the placeholder inside the image, but rather use a background (or placeholder) that the image is overlaid upon. The placeholder component is intended to be a "faster" solution for those cases where you do not have any kind of intrinsic (box preserving the aspect ratio) component.

Fixes:

• Optional properties are properly marked as such in the types

Breaking:

• The ({cls}) render prop argument has been removed
  The cls property was not adding much, since we are not using it
  as a hook for styling in the library. There is an argument that
  we could use it for fallbacks, but that requires the user to
  set the style tag anyway. Thus, it is best to leave it undefined.
  It also bloated the syntax quite a bit, with a mess of squgglies
  that can easily be messed up; JSX prop, arrow function, and then
  destructuring. {({cls}) => ... } vs {() => ..}. I think this
  is more convenient for people to use, especially weighed against
  the lack of benefit to the library.