Thanks for reporting your issue. Please make sure these boxes are checked before submitting your issue - thank you!

- [x] I looked for a similar issue and couldn't find any.
- [x] I tried with the latest version of GPAC. Installers available at http://gpac.io/downloads/gpac-nightly-builds/
- [x] I give enough information for contributors to reproduce my issue (meaningful title, github labels, platform and compiler, command-line ...). I can share files anonymously with this dropbox: https://www.mediafire.com/filedrop/filedrop_hosted.php?drop=eec9e058a9486fe4e99c33021481d9e1826ca9dbc242a6cfaab0fe95da5e5d95

Detailed guidelines: http://gpac.io/2013/07/16/how-to-file-a-bug-properly/

Actually i'm editing MPD file after creation to add file.vtt to load it with dash.js:
```
    <AdaptationSet id="3" contentType="text" mimeType="text/vtt" lang="fre" startWithSAP="1">
      <Label>Français forcés</Label>
      <Role schemeIdUri="urn:mpeg:dash:role:2011" value="forced-subtitle"/>
      <Representation id="3" bandwidth="256">
        <BaseURL>3_fre.vtt</BaseURL>
      </Representation>
    </AdaptationSet>
```
I was hopping MP4Box can do it from command line, for example:

```
MP4Box -dash 4000 -frag 4000 -rap -fps 24 \
test.mp4#trackID=1:id=video \
test.mp4#trackID=2:id=fre \
/path/to/3_fre.vtt#subtitle:id=3:asID=3:role=forced-subtitle:desc_as='<Label>Français forcés</Label>' \
-out index.mpd
```
Thank you.

Feature request: possibility to add a file.vtt to MPD from command line
This is a GitHub Issue
repo:gpac
owner:gpac
Title : Feature request: possibility to add a file.vtt to MPD from command line
Issue date:

--- start body ---
Thanks for reporting your issue. Please make sure these boxes are checked before submitting your issue - thank you!

- [x] I looked for a similar issue and couldn't find any.
- [x] I tried with the latest version of GPAC. Installers available at http://gpac.io/downloads/gpac-nightly-builds/
- [x] I give enough information for contributors to reproduce my issue (meaningful title, github labels, platform and compiler, command-line ...). I can share files anonymously with this dropbox: https://www.mediafire.com/filedrop/filedrop_hosted.php?drop=eec9e058a9486fe4e99c33021481d9e1826ca9dbc242a6cfaab0fe95da5e5d95

Detailed guidelines: http://gpac.io/2013/07/16/how-to-file-a-bug-properly/

Actually i'm editing MPD file after creation to add file.vtt to load it with dash.js:
```
    <AdaptationSet id="3" contentType="text" mimeType="text/vtt" lang="fre" startWithSAP="1">
      <Label>Français forcés</Label>
      <Role schemeIdUri="urn:mpeg:dash:role:2011" value="forced-subtitle"/>
      <Representation id="3" bandwidth="256">
        <BaseURL>3_fre.vtt</BaseURL>
      </Representation>
    </AdaptationSet>
```
I was hopping MP4Box can do it from command line, for example:

```
MP4Box -dash 4000 -frag 4000 -rap -fps 24 \
test.mp4#trackID=1:id=video \
test.mp4#trackID=2:id=fre \
/path/to/3_fre.vtt#subtitle:id=3:asID=3:role=forced-subtitle:desc_as='<Label>Français forcés</Label>' \
-out index.mpd
```
Thank you.

--- end body ---

None