Add an "onClick" field to submit button

Hey, Brandon,

Love the plugin. I use it across a couple of different Wordpress installs -- great stuff, thanks!

I wanted to track clicks on different mailing list signup forms across the site, to see what is working and what isn't. The easiest way to do this was to use Google Events tracking via an onClick parameter. So I hacked together a version of the plugin that supports that.

Concretely:
![screen shot 2014-09-12 at 11 35 42 pm](https://cloud.githubusercontent.com/assets/221121/4259588/7d241b28-3aff-11e4-8971-c7e61beb9832.png)

And this changes the generated HTML to this:

     <input value="Join Us" id="subbutton" onclick="ga('send', 'event', 'Button', 'OptIn', 'Sidebar - Top');" type="submit">

Not sure if this is something you're looking to add support for, but I figured I'd submit the pull request anyways. Oh, and my PHP skills are completely limited to what I've learned breaking WordPress, so, uh, buyer beware.