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/pattern.go

@@ -403,6 +403,8 @@ func (p *Pattern) transformInput(item *Item) []Token {
 
 	tokens := Tokenize(item.text.ToString(), p.delimiter)
 	ret := Transform(tokens, p.nth)
+	// TODO: We could apply StripLastDelimiter to exclude the last delimiter from
+	// the search allowing suffix match with a string or a regex delimiter.
 	item.transformed = &transformed{p.revision, ret}
 	return ret
 }