Address outstanding issues

[jacob7395]

Initial draft to allow users to set the playback speed for an audio clip. Also put a filter in to ensure only "valid" bookmarks are displayed in the list.

I welcome any feedback.

Additionality I have resolved the following issues:

10 by adding a playback setting

21 by adding a loop setting

18 by adding an audio toggle key bind

6 by adding a bookmark button

Example Settings

```audio-player
[[my awesome audio file.mp3]]
00:00:31 --- test
playback: 1
loop: true
```

[noonesimg]

Hey! First of all, thanks a lot for the contribution! I haven't had much time to address these issues, so it's really good to see someone else helping with that :)

I've looked through the code, here's what think:

1. The "add bookmark" button - it's only visible on a regular screen, there's no way to access it on the mobile, where it is much more needed.
2. The commands work just fine, although would be cool to also have a couple of buttons to change these settings, without going into the text editor mode.

Let me know if you're interested in implementing these updates.

[noonesimg]

also, just a sidenote, but don't use var, it's better to go with const or let I feel like that might be a simple typo on your side, because there's only one case of var in the getPlaybackSpeedSetting() method

[jacob7395]

Sorry for the slow reply been back at work this week. I'll work on your comments now, I'm bad at styling but I'll try to get some basic buttons in to edit the playback speed and loop setting. We can improve the styling in this PR or another time.

[noonesimg]

No, worries, there's no rush. Good to hear! I'm not that good with styling either. But I guess we'll figure something out :)