Create option to automatically add width and height to img

[andreisergiu98]

Since we have these values i thought it my be useful to add them automatically to <img>. I know that sources will most likely have different values, but the ratio should be the same. Also when browsers will add support for width and height in sources we can easily extend this.

I created another pull request because https://github.com/cyrilwanner/react-optimized-image/pull/11 has unsigned commits.