Make warnings more visible by coloring them

Warnings are currently easy to miss because they are visually indistinguishable
from other text. It would be better if we write 'Warning:' in bold
and color it in yellow, similar to deprecation warnings from the depd
package.

* If `process.stdout` is a terminal emulator: Use ANSI escape codes
to switch the text weight and color to bold and yellow, write 'Warning:' and
switch text style back to normal.
* Otherwise: Write the string 'Warning:' as usual.

ANSI escape codes are described here:
https://en.wikipedia.org/wiki/ANSI_escape_code

lib/convict.js

@@ -635,7 +635,14 @@ let convict = function convict(def) {
         let output_err_bufs = [types_err_buf, missing_err_buf];
 
         if (options.allowed === ALLOWED_OPTION_WARN && params_err_buf.length) {
-          global.console.log('Warning: '+  params_err_buf);
+          let warning = 'Warning:';
+          if (process.stdout.isTTY) {
+            // Write 'Warning:' in bold and in yellow
+            const SET_BOLD_YELLOW_TEXT = '\x1b[33;1m';
+            const RESET_ALL_ATTRIBUTES = '\x1b[0m';
+            warning = SET_BOLD_YELLOW_TEXT + warning + RESET_ALL_ATTRIBUTES;
+          }
+          global.console.log(warning + ' ' + params_err_buf);
         } else if (options.allowed === ALLOWED_OPTION_STRICT) {
           output_err_bufs.push(params_err_buf);
         }