Is there a way to tell Sublime to highlight comments that contain the word "TODO"?
I've looked for plugins that do this but haven't really been able to find much.
Sublime Linter should do the trick. You can add underlining for custom static text.
All you need is the Sublime Package Manager:
As stated by jon in the first comment:
For ST3, the SublimeLinter-annotations plugin highlights TODO, FIXME etc.