Allow an images width to be set

Add !img-wNNN to image handling so a width can be specified for an
image.
Add acceptance test and documentation for !img-w

Add documentation and an acceptance test to cover !img-w keyword
Change syntax to !img -w nnn url

Changed syntax of including width to be !img -w nnn url as suggested by
@jediwhale so it can be used in all image type markup
Add border and margin attributes

Add the ability to specify border and margin for an image using !img -b
x and !img -m y
Merge pull request #187 from markhop/image-attributes

Allow an images width to be set