Tables in the plain-text target, with captions

A @table now renders to txt.  What it needed settled how plain text sets
a block off from the prose around it, which is by INDENTING it: that is
the only structural device the medium has, where html and LaTeX have
boxes, rules and space.  An indented block is one that has already been
laid out, and the target leaves its lines alone.

A caption is placed above a table or a listing and below a figure, in
every target rather than only in plain text.  A figure is taken in at a
glance, and the caption then names what the reader has already seen; a
table is read, and a reader who meets the rows before the subject has to
read them twice.

Also fixed, all three of them silent: an indented paragraph was dropped
from plain-text output rather than left as written; an @indent block was
re-flowed, so the indentation of its later lines appeared as gaps in the
middle of a line; and a table captioned with ":number false" lost the
rule under its caption, while an unnumbered table wrongly consumed a
number that a reader counting captions would then find missing.

(from dev 8173c371bba6)

sks/target/target.k

@@ -36,6 +36,10 @@ of this: neither the transforms nor the quoting reach it.
   :value 72
 @@@
 
+@@@state Target_txt_indent
+  :desc Width of indentation in the plain text target (txt)
+  :value 3
+@@@
 
 # The txt target is not fully implemented in the SKS yet.
 @@@target txt | Plain text with formatting |  
@@ -48,6 +52,7 @@ of this: neither the transforms nor the quoting reach it.
   ~   ^0020^
   :after_apply
     phases.txt_format_footnotes ;
+    phases.txt_insert_captions ;
     phases.txt_justify_blocks
 @@@