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_cfgable.h

@@ -276,6 +276,7 @@ struct GlobalConfig {
   int progressmode;               /* CURL_PROGRESS_BAR / CURL_PROGRESS_STATS */
   char *libcurl;                  /* Output libcurl code to this file name */
   bool fail_early;                /* exit on first transfer error */
+  bool styled_output;             /* enable fancy output style detection */
   struct OperationConfig *first;
   struct OperationConfig *current;
   struct OperationConfig *last;   /* Always last in the struct */