This is a simplistic plugin implementing GameJolt API. It includes auth, achievement unlocking, and score posting, all with according callbacks.
As simple as it goes - unzip folder to "Construct 2\exporters\html5\plugins" directory, and restart IDE. Plugin will appear under "Platform specific" category on next launch.
Include GameJolt plugin into your project by adding an object of it (it is global, so you only need to do this once). Set up your game ID and private key in object properties (this data can be discovered on dashboard). Afterwards, add according events and actions where appropriate.
Note that while the plugin will automatically authenticate the user, it can take a second or two to do that, so for "first visit" actions you should use "Logged in" event.
Announcement blog post:
Update 1 blog post:
Demo source (.capx):
If there are any questions, feel free to ask.