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/util/chars.go

@@ -189,6 +189,27 @@ func (chars *Chars) TrimTrailingWhitespaces() {
 	chars.slice = chars.slice[0 : len(chars.slice)-whitespaces]
 }
 
+func (chars *Chars) TrimSuffix(runes []rune) {
+	lastIdx := len(chars.slice)
+	firstIdx := lastIdx - len(runes)
+	if firstIdx < 0 {
+		return
+	}
+
+	for i := firstIdx; i < lastIdx; i++ {
+		char := chars.Get(i)
+		if char != runes[i-firstIdx] {
+			return
+		}
+	}
+
+	chars.slice = chars.slice[0:firstIdx]
+}
+
+func (chars *Chars) SliceRight(last int) {
+	chars.slice = chars.slice[:last]
+}
+
 func (chars *Chars) ToString() string {
 	if runes := chars.optionalRunes(); runes != nil {
 		return string(runes)