ox-ascii: Implement "justifyright" and "justifyleft" blocks

ox-ascii: Implement "justifyright" and "justifyleft" blocks

* lisp/ox-ascii.el
(org-ascii--justify-lines): New function.  Renamed from
`org-ascii--justify-string'.
(org-ascii--justify-element, org-ascii--current-justification): New
functions.
(org-ascii-verse-block, org-ascii-table-cell, org-ascii-table
  org-ascii-src-block, org-ascii-property-drawer, org-ascii-planning,
  org-ascii-paragraph, org-ascii-paragraph,
  org-ascii-latex-environment, org-ascii-keyword,
  org-ascii-horizontal-rule, org-ascii-fixed-width,
  org-ascii-export-block, org-ascii-example-block, org-ascii-clock,
  org-ascii-template--document-title,
  org-ascii-template--document-title): Use new functions.
(org-ascii-center-block): Do not process contents anymore since they
are already justified.

* doc/org.texi (ASCII/Latin-1/UTF-8 export): Document new feature.
  Also document `org-ascii-text-width' since this is closely related.

This patches also fixes centering of tables.