Add 'transform' action to conditionally perform a series of actions

'transform' action runs an external command that prints a series of
actions to perform.

  # Disallow selecting an empty line
  echo -e "1. Hello\n2. Goodbye\n\n3. Exit" |
    fzf --reverse --header 'Select one' \
        --bind 'enter:transform:[[ -n {} ]] && echo accept || echo "change-header:Invalid selection"'

  # Move cursor past the empty line
  echo -e "1. Hello\n2. Goodbye\n\n3. Exit" |
    fzf --reverse --header 'Select one' \
        --bind 'enter:transform:[[ -n {} ]] && echo accept || echo "change-header:Invalid selection"' \
        --bind 'focus:transform:[[ -n {} ]] && exit; [[ {fzf:action} =~ up$ ]] && echo up || echo down'

Close #3368
Close #2980

man/man1/fzf.1

@@ -574,10 +574,6 @@ e.g.
 When using a field index expression, leading and trailing whitespace is stripped
 from the replacement string. To preserve the whitespace, use the \fBs\fR flag.
 
-Also, \fB{q}\fR is replaced to the current query string, and \fB{n}\fR is
-replaced to zero-based ordinal index of the line. Use \fB{+n}\fR if you want
-all index numbers when multiple lines are selected.
-
 A placeholder expression with \fBf\fR flag is replaced to the path of
 a temporary file that holds the evaluated list. This is useful when you
 multi-select a large number of items and the length of the evaluated string may
@@ -589,6 +585,15 @@ e.g.
      seq 100000 | fzf --multi --bind ctrl-a:select-all \\
                       --preview "awk '{sum+=\\$1} END {print sum}' {+f}"\fR
 
+Also,
+
+* \fB{q}\fR (or \fB{fzf:query}\fR) is replaced to the current query string
+.br
+* \fB{n}\fR is replaced to the zero-based ordinal index of the current item.
+  Use \fB{+n}\fR if you want all index numbers when multiple lines are selected.
+.br
+* \fB{fzf:action}\fR is replaced to to the name of the last action performed
+
 Note that you can escape a placeholder pattern by prepending a backslash.
 
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