fix: Support for .mjs es6 modules (#1138)

Author: JayBeavers

README.md

@@ -155,7 +155,7 @@ Don't use unix globbing to pass multiple directories, e.g `--watch ./lib/*`, it
 
 ## Specifying extension watch list
 
-By default, nodemon looks for files with the `.js`, `.coffee`, `.litcoffee`, and `.json` extensions. If you use the `--exec` option and monitor `app.py` nodemon will monitor files with the extension of `.py`. However, you can specify your own list with the `-e` (or `--ext`) switch like so:
+By default, nodemon looks for files with the `.js`, `.mjs`, `.coffee`, `.litcoffee`, and `.json` extensions. If you use the `--exec` option and monitor `app.py` nodemon will monitor files with the extension of `.py`. However, you can specify your own list with the `-e` (or `--ext`) switch like so:
 
     nodemon -e js,jade
 

doc/cli/help.txt

@@ -23,7 +23,7 @@
   -- <your args> ........... to tell nodemon stop slurping arguments.
 
   Note: if the script is omitted, nodemon will try to read "main" from
-  package.json and without a nodemon.json, nodemon will monitor .js, .coffee,
+  package.json and without a nodemon.json, nodemon will monitor .js, .mjs, .coffee,
   and .litcoffee by default.
 
   To learn more about nodemon.json config: nodemon --help config

lib/config/exec.js

@@ -141,7 +141,7 @@ function exec(nodemonOptions, execMap) {
   if (options.exec === 'coffee') {
     // don't override user specified extension tracking
     if (!options.ext) {
-      extension = 'coffee,litcoffee,js,json';
+      extension = 'coffee,litcoffee,js,json,mjs';
     }
 
     // because windows can't find 'coffee', it needs the real file 'coffee.cmd'