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/terminal.go

@@ -305,6 +305,7 @@ type Terminal struct {
 	nthAttr            tui.Attr
 	nth                []Range
 	nthCurrent         []Range
+	acceptNth          []Range
 	tabstop            int
 	margin             [4]sizeSpec
 	padding            [4]sizeSpec
@@ -914,6 +915,7 @@ func NewTerminal(opts *Options, eventBox *util.EventBox, executor *util.Executor
 		nthAttr:            opts.Theme.Nth.Attr,
 		nth:                opts.Nth,
 		nthCurrent:         opts.Nth,
+		acceptNth:          opts.AcceptNth,
 		tabstop:            opts.Tabstop,
 		hasStartActions:    false,
 		hasResultActions:   false,
@@ -1561,16 +1563,24 @@ func (t *Terminal) output() bool {
 	for _, s := range t.printQueue {
 		t.printer(s)
 	}
+	transform := func(item *Item) string {
+		return item.AsString(t.ansi)
+	}
+	if len(t.acceptNth) > 0 {
+		transform = func(item *Item) string {
+			return JoinTokens(StripLastDelimiter(Transform(Tokenize(item.AsString(t.ansi), t.delimiter), t.acceptNth), t.delimiter))
+		}
+	}
 	found := len(t.selected) > 0
 	if !found {
 		current := t.currentItem()
 		if current != nil {
-			t.printer(current.AsString(t.ansi))
+			t.printer(transform(current))
 			found = true
 		}
 	} else {
 		for _, sel := range t.sortSelected() {
-			t.printer(sel.item.AsString(t.ansi))
+			t.printer(transform(sel.item))
 		}
 	}
 	return found
@@ -3847,7 +3857,7 @@ func replacePlaceholder(params replacePlaceholderParams) (string, []string) {
 				elems, prefixLength := awkTokenizer(params.query)
 				tokens := withPrefixLengths(elems, prefixLength)
 				trans := Transform(tokens, nth)
-				result := joinTokens(trans)
+				result := JoinTokens(trans)
 				if !flags.preserveSpace {
 					result = strings.TrimSpace(result)
 				}
@@ -3897,7 +3907,7 @@ func replacePlaceholder(params replacePlaceholderParams) (string, []string) {
 			replace = func(item *Item) string {
 				tokens := Tokenize(item.AsString(params.stripAnsi), params.delimiter)
 				trans := Transform(tokens, ranges)
-				str := joinTokens(trans)
+				str := JoinTokens(trans)
 
 				// trim the last delimiter
 				if params.delimiter.str != nil {