Add three-state repeat button

Add a new button to the controls that defines whether the media repeats
at the end of the stream. The states are:

- Disabled: Media does not repeat. This is existing behavior and the
default state.
- Always: Media always repeats.
- Once: Media repeats once. Flag is reset when loading new media.

Tested all three states with both audio and video.

Added a `rustfmt.toml` to avoid reordering imports. They keep moving
around in previous pull requests, best to just define what we want in
the repository.

Fixes: #39

rustfmt.toml

@@ -0,0 +1 @@
+reorder_imports = false