Fixes #34: Add --images parameter for overriding images.

Users are now able to provide a directory, using the --images parameter,
to override images from the original books with their own modified or
branded versions.

The directory structure and file names of images within the provided
directory must match that of the target.

For instance to override the common image user_image_create.png I would
create the image in this folder:

/home/user/Pictures/images/common/user_image_create.png

I would then run openstack-manuals-convert with:

openstack-manuals-convert --images=/home/user/Pictures/images

The image would eventually land in:

./target/publican/en-US/images/common/user_image_create.png