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

src/core.go

@@ -128,7 +128,7 @@ func Run(opts *Options) (int, error) {
 				}
 			}
 			trans := Transform(tokens, opts.WithNth)
-			transformed := joinTokens(trans)
+			transformed := JoinTokens(trans)
 			if len(header) < opts.HeaderLines {
 				header = append(header, transformed)
 				eventBox.Set(EvtHeader, header)