add --watch tag (apidoc#835)

fix apidoc#833

and remove Node v6 from Travis CI

Author: omargho

.travis.yml

@@ -1,6 +1,9 @@
 language: node_js
 sudo: false
 node_js:
-  - 6
+  - 12
+  - 11
+  - 10
+  - 8
 before_script:
   - npm install -g grunt-cli

bin/apidoc

@@ -13,6 +13,7 @@
 
 var path   = require('path');
 var cmd    = require('commander');
+var nodemon = require('nodemon');
 var apidoc = require('../lib/index');
 
 var argv = cmd
@@ -57,6 +58,8 @@ var argv = cmd
 
     .option('--filter-by <tag-filter=value>', 'Filter documentation by tag')
 
+    .option('--watch', 'Watch the Input/Source dirname for changes to rebuild the docs')
+
     .parse(process.argv)
 ;
 
@@ -120,6 +123,19 @@ var options = {
     filterBy      : argv.filterBy,
 };
 
+if (argv.watch) {
+  nodemon({watch: options.src});
+  console.log('Watching mode is Activated\nListening for changes...');
+  nodemon.on('quit', function () {
+    console.log('ApiDoc has exited');
+    process.exit()
+  }).on('restart', function () {
+    console.log('Changes detected, Rebuilding the docs...');
+    apidoc.createDoc(options);
+    console.log('Listening for changes...');
+  });
+}
+
 if (apidoc.createDoc(options) === false) {
   process.exit(1);
 }