broscript

broscript.js

(function() {
    "use strict";
    var chat = document.getElementById('chat');
 
    function parseNode(node) {
        if (node.classList && node.classList.contains('message') && !node.classList.contains('pending') &&
            !node.querySelector('.onebox') && !node.querySelector('pre') ) {
            node.textContent =  node.textContent.replace(/[\s\!\?\.]+$/, '') + ', bro.';
        }
    }
    [].forEach.call(chat.querySelectorAll('.user-container .message'), parseNode);
    new MutationObserver(function(records) {
        records.forEach(function(record) {
            [].forEach.call(record.addedNodes, parseNode);
        });
    }).observe(chat, {
        childList: true,
        subtree: true
    });
}());

Now with more sugar bro!

(function() {
    "use strict";
    var chat = document.getElementById('chat');
    function parseNode(node) {
        if (node.classList && node.classList.contains('message') && !node.classList.contains('pending') &&
            !node.querySelector('.onebox') && !node.querySelector('pre') ) {
            node.textContent = node.textContent.replace(/((?:^|\s*)(?:[a-z'-]+))([\?\!\.]+$|[\?\!\.]+\s|\s*:.|\s*$)/gi, '$1 bro$2') + '';
       }
    }
    [].forEach.call(chat.querySelectorAll('.user-container .message'), parseNode);
    new MutationObserver(function(records) {
        records.forEach(function(record) {
            [].forEach.call(record.addedNodes, parseNode);
        });
    }).observe(chat, {
        childList: true,
        subtree: true
    });
}());

EDIT: Updated RegEx