Support horizontal alignment for AsciiDoc table cells

[mojavelinux]

Currently, the horizontal alignment on an AsciiDoc table cell is ignored. Consider the following case:

[cols=<1a]
|===
|top right
|===

The text will be displayed on the left of the cell instead of the right.

This limitation is a consequence of how AsciiDoc table cells are implemented in Asciidoctor PDF. The content is rendered as individual child blocks inside the cell container.

However, given that most of the time a non-left alignment is specified for cells that only contain paragraph content, it's possible to support the horizontal alignment in this case by temporary modifying the base text align setting. The horizontal alignment should be honored in this case.