Disable automatic external link styling when link is an image

[eric-schneider]

NOTE: This issue should probably be worked on in conjunction with #120.

In AsciiDoc, you can add links to images. However, when the link is an external link, the automatic external link styling implemented in Automatic External Links applies a box arrow icon to the image. This is undesirable, and we should update the CSS to ignore external links when they're a part of an image.

[gopalchobey9]

@eric-schneider please provide the file name to be updated