Add option "force download" to playback formats

[mconf-daileon]

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Author Name: Leonardo Daronco (Leonardo Daronco)
Original Redmine Issue: 1538, http://dev.mconf.org/redmine/issues/1538

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There should be an option in the playback formats to indicate that they are formats that should be downloaded instead of played in the browser. This is useful for formats that, for instance, generate a video file or an export of the HTML5 playback page.
Having a flag that indicates that the playback format is a download format, applications can then adapt the tags and attributes that are generated for these formats.