Update code throughout linterReporter for better style and performance.

Mostly fixes from previous PRs of mine. Some (seemingly) better comments and ways of doing things.

@MiguelCastillo: I'll add comments on the lines where I want to better document why the change was done.
No need to check if it exists since we created it above if it didn't:
```javascript
if (!mark) {
    mark = {
      // ...
     warnings: [],
     errors: [],
     // ....
    }
}
```