Allow comment_form() to add attributes to <form> tag

[psbook]

The standard method to allow Google Analytics to track comment submissions as a 'goal' is to add a piece of JavaScript code wrapped in an 'onsumbit' attribute:
​http://www.optimisationbeacon.com/analytics/track-blog-comments-in-google-analytics/
​http://code.google.com/apis/analytics/docs/tracking/eventTrackerGuide.html

We want something like the following:

<form action="http://www.example.com/wp-comments-post.php" method="post" id="commentform" onsubmit="_gaq.push(['_trackEvent', 'Comments', 'Submit', 'POST TITLE']);">

However the current arguments submitted to comment_form() do not provide for adding attributes to <form>
​http://codex.wordpress.org/Function_Reference/comment_form

I was forced to perform a workaround in which the onsubmit code was inserted by JavaScript on the fly at runtime.

So this is request to alter comment_form() such that arguments accepted provide for 'onsubmit' or other custom attributes to be appended on the <form> tag.

The proposed/attached patch adds a 'form_attributes' argument to the $args for comment_form, it also adds 2 new filters, one for the attributes (which are passed as a key/value array) and one for the entire form element as it gets output.

initial path had syntax errors, now fixed

Version field indicates when the enhancement was initially suggested.