curl: added --styled-output

It is enabled by default, so --no-styled-output will switch off the
detection/use of bold headers.

Closes #2538

src/tool_help.c

@@ -414,6 +414,8 @@ static const struct helptxt helptext[] = {
    "Use SSLv3"},
   {"    --stderr",
    "Where to redirect stderr"},
+  {"    --styled-output",
+   "Enable styled output for HTTP headers"},
   {"    --suppress-connect-headers",
    "Suppress proxy CONNECT response headers"},
   {"    --tcp-fastopen",