feat: Support WMA files, including those with embedded album art (#143)

The tag library supports WMA, so add the mimetype.

Some wma files have embedded album art encoded as a video stream
alongside the audio:

```
Input #0, asf, from '01. Emergency Pulloff.wma':
  Metadata:
    (snip)
  Duration: 00:03:11.47, start: 0.000000, bitrate: 129 kb/s
    Stream #0:0: Video: mjpeg, yuvj420p(pc, bt470bg/unknown/unknown), 200x200 [SAR 96:96 DAR 1:1], 90k tbr, 90k tbn, 90k tbc
    Metadata:
      comment         : Cover (front)
    Stream #0:1: Audio: wmav2 (a[1][0][0] / 0x0161), 44100 Hz, stereo, fltp, 128 kb/s
Output #0, opus, to '/tmp/output.opus':
Output file #0 does not contain any stream
```

The `-map 0:0` passed to ffmpeg is selecting the video stream to
transcode, then `-vn` says not to transcode video, so the whole process
returns an error code:

```
2021/05/28 18:59:09 transcoding according to transcoding profile of 96k
2021/05/28 18:59:09 serving transcode `02. Cotton Patch Rag.wma`: cache [opus/96k] miss!
2021/05/28 18:59:09 serving transcode `02. Cotton Patch Rag.wma`: error: starting transcode: running ffmpeg: exit status 1
```

I believe the correct solution here is to use an _audio_ stream
specifier, as in `-map 0🅰️0`.  Doing this selects the audio and
successfully performs the transcode.

server/encode/encode.go

@@ -88,7 +88,7 @@ func ffmpegCommand(filePath string, profile Profile) (*exec.Cmd, error) {
 	args := []string{
 		"-v", "0",
 		"-i", filePath,
-		"-map", "0:0",
+		"-map", "0:a:0",
 		"-vn",
 		"-b:a", fmt.Sprintf("%dk", profile.Bitrate),
 	}