feat(img): CSS Backgrounds for non-IMG tags

The phImg directive will now set the `src` attribute only for `img`
tags. For all other tags, it will set the CSS `background-image`
property to `URL(dataURL)`.

Closes #5.
Merge pull request #9 from joshdmiller/img-css-background

feat(img): CSS Backgrounds for non-IMG tags

The phImg directive will now set the src attribute only for img
tags. For all other tags, it will set the CSS background-image
property to URL(dataURL).

Closes #5.