Add --accept-nth option to transform the output

This option can be used to replace a sed or awk in the post-processing step.

  ps -ef | fzf --multi --header-lines 1 | awk '{print $2}'
  ps -ef | fzf --multi --header-lines 1 --accept-nth 2

This may not be a very "Unix-y" thing to do, so I've always felt that fzf
shouldn't have such an option, but I've finally changed my mind because:

* fzf can be configured with a custom delimiter that is a fixed string
  or a regular expression.
* In such cases, you'd need to repeat the delimiter again in the
  post-processing step.
* Also, tools like awk or sed may interpret a regular expression
  differently, causing mismatches.

You can still use sed, cut, or awk if you prefer.

Close #3987
Close #1323

man/man1/fzf.1

@@ -120,6 +120,10 @@ fields.
 .BI "\-\-with\-nth=" "N[,..]"
 Transform the presentation of each line using field index expressions
 .TP
+.BI "\-\-accept\-nth=" "N[,..]"
+Define which fields to print on accept. The last delimiter is stripped from the
+output.
+.TP
 .B "+s, \-\-no\-sort"
 Do not sort the result
 .TP