Cannot handle multiple languages and subtitles

[EyMaddis]

This is a duplication of the following issue:
videojs/http-streaming#645

A playlist like this is not properly parsed:

    <AdaptationSet mimeType="text/vtt" lang="de">
      <Role schemeIdUri="urn:mpeg:dash:role" value="subtitle"/>
      <Label>Deutsch</Label>
      <Representation id="4" bandwidth="256">
        <BaseURL>dashcaption_de_dash.mpd</BaseURL>
      </Representation>
    </AdaptationSet>
    <AdaptationSet mimeType="text/vtt" lang="de">
      <Role schemeIdUri="urn:mpeg:dash:role" value="subtitle"/>
      <Label>Forced</Label>
      <Representation id="5" bandwidth="256">
        <BaseURL>dashcaption_forced_dash.mpd</BaseURL>
      </Representation>
    </AdaptationSet>```

only the first `de` will be available in the result.

I think this boils down to [this line](https://github.com/videojs/mpd-parser/blob/12387495d2fe472eab2fdd7d0c7f1b4bbc22c7cb/src/toM3u8.js#L181).

**The same applies to audio tracks.**

[LienLotus]

I have same issue with audio tracks

[gkatsev]

Another example file from #95 https://github.com/videojs/mpd-parser/files/4449794/manifest.txt