Add 'r' flag (raw) for unquoted output

By default, placeholder expressions are automatically quoted to ensure
they are safely passed as arguments to external programs.

The r flag ({r}, {r1}, etc.) disables this behavior, outputting the
evaluated value without quotes.

For example,

  echo 'foo   bar' | fzf --preview 'echo {} {r}'

The preview command becomes:

  echo 'foo   bar' foo   bar

Since `{r}` expands to unquoted "foo   bar", 'foo' and 'bar' are passed
as separate arguments.

**Use with caution** Unquoted output can lead to broken commands.

  echo "let's go" | fzf --preview 'echo {r}'

Close #4330

man/man1/fzf.1

@@ -768,6 +768,12 @@ e.g.
      \fBfzf \-\-multi \-\-preview='head \-10 {+}'
      git log \-\-oneline | fzf \-\-multi \-\-preview 'git show {+1}'\fR
 
+Each expression expands to a quoted string, so that it's safe to pass it as an
+argument to an external command. So you should not manually add quotes around
+the curly braces. But if you don't want this behavior, you can put
+\fBr\fR flag (raw) in the expression (e.g. \fB{r}\fR, \fB{r1}\fR, etc).
+Use it with caution as unquoted output can lead to broken commands.
+
 When using a field index expression, leading and trailing whitespace is stripped
 from the replacement string. To preserve the whitespace, use the \fBs\fR flag.