Build: Add /.github, /build & /external to .gitignore

Also, make some `.gitignore` paths absolute.

Fixes jquerygh-2075

Author: mgol

.npmignore

@@ -1,10 +1,14 @@
-demos
+/.github
+/build
+/demos
 /dist/cdn
-tests
+/external
+/tests
+/Gruntfile.js
+
+.csslintrc
 .editorconfig
 .eslintrc.json
 .eslintignore
-.mailmap
-Gruntfile.js
-.csslintrc
 .gitattributes
+.mailmap