background.js

(function () {
  const ELEMENT_NODE = 1;
  const TEXT_NODE = 3;
  // https://stackoverflow.com/a/34559316

  function walkText(node) {
    if (node.nodeType == TEXT_NODE) {
      node.data = node.data.replace(/(?<=[ \t]|(?:^|\"|\-|\=|(?:[.!?]\s)))(Sean|sean)(?=([ \t]|\.|\'||\-|\=|\"|\?|\/|\,))/g, "Shane");
    } else if (node.nodeType == ELEMENT_NODE && node.nodeName != "SCRIPT") {
      for (var i = 0; i < node.childNodes.length; i++) {
        walkText(node.childNodes[i]);
      }
    }
  }
  
  walkText(document.body);

  document.title = document.title.replace(/(?<=[ \t]|(?:^|\"|\-|\=|(?:[.!?]\s)))(Sean|sean)(?=([ \t]|\.|\'||\-|\=|\"|\?|\/|\,))/g, "Shane");
})();

gistfile1.txt

This is a super simple example of a chrome extension that runs on every page. 
It converts all instance of the word Sean to Shane.
I made it because our nickname for our roommate Sean is Shane.

If you want it to run on only specific pages, modify the "matches" part of the manifest.json to the URL.
Ex: 
// Only run on the GitHub pull request page
// https://developer.chrome.com/extensions/match_patterns
"matches":[
  "*://github.com/org_name/repo_name/pull/*"
],

To install:
- Save the manifest.json and background.json to a folder
- open chrome://extensions
- enable Developer mode at the top right
- click on "Load unpacked" 
- select the folder

manifest.json

  {
    "name": "2Shanez",
    "version": "1.0",
    "description": "Sean 2 Shane",
    "manifest_version": 2,
    "persistent": false,
    "content_scripts": [
      {
        "matches": [
          "<all_urls>"
        ],
        "js": [
          "background.js"
        ],
        "run_at": "document_end"
      }
    ]
  }