Add user-defined tags/annotations to replays

[davidalayachew]

Add the ability for users to add an annotation/tag to replays. Maybe I want to highlight that a particular replay did something cool offstage, so I could add an "OFFSTAGE" annotation or something. Then, if #179 ever comes out, we could add filtering by annotations.

Right now, my workaround is just to use the file name to highlight the unique things in the replay. Annotations would work better.