Alignment of block image should be controlled using block align roles instead of text align roles

[mojavelinux]

Currently, the converter looks for a role with the prefix text- (e.g., text-center) to align a block image if neither the float or align attributes are set. However, these roles are intended to be used for aligning text. The converter should instead look for the block align roles (e.g., center).

Example:

image::center-me.png[role=center]