Escaping, table layout, and document fixes

Quoted Klammertext specials (^@ ^| ^# ^^ ^: ^*) and ^'...'^ regions now
survive re-processing (held as escape markers until final output);
:after_apply phase functions receive and return raw target text.

Tables: :hpos element position (center|left|right|<length>) replaces the
unimplemented :center/:indent; the ranged cell override is renamed
:justify; :column_width works in html (colgroup widths) and gains
'fill' -- the remaining width, capped at the column's widest entry, in
both targets; a table wider than the text column warns on the console;
table edges without an outer line set their text flush on the margins.

@document: no empty title bar for untitled documents; @vfill fills to
the bottom of the window in html (pure CSS); @vspace in plain text;
new @dot klammer; monospace email links.

sks/target/css/target.css

@@ -160,6 +160,14 @@
     display: inline-flex;
 }
 
+/* A length :hpos; the margin-left comes as an inline style. */
+.hpos_indent {
+    display: flex;
+    flex-direction: row;
+    align-items: center;
+    justify-content: left;
+}
+
 .hpos_margin {
     margin: 0 1em 0 1em;
 }