Support VLC's start-time / stop-time extension to playlist

[FelipeLema]

Description
Support start-time and stop-time extension in playlist. This allows users to build more complex playlists. For my case, to inject loops into videogame music using https://github.com/arkrow/PyMusicLooper

• start-time describes a time where the item should start.
• stop-time describes a time where the item should stop once it has been reached.
• Both use seconds as unit and a floating point number can be used to specify fractions of a second (ie: 6.5s = 6seconds + 500milliseconds).

Not leaning towards m3u or xspf. Could be either.

m3u sample:

#EXTM3U
#EXTINF:617,School.mkv
#EXTVLCOPT:start-time=1
#EXTVLCOPT:stop-time=167
School.mkv
#EXTVLCOPT:start-time=291
#EXTVLCOPT:stop-time=617
School.mkv

xpsf sample:

 <?xml version="1.0" encoding="UTF-8"?>
 <playlist version="1" xmlns="http://xspf.org/ns/0/" xmlns:vlc="http://www.videolan.org/vlc/playlist/ns/0/">
    <title>Playlist</title>
    <location>D:/media/example.xspf</location>
    <trackList>
        <track>
            <title>DVD seconds 42 to 45 muted</title>
            <location>dvd://d:\@1</location>
            <extension application="http://www.videolan.org/vlc/playlist/0">
                <vlc:id>3</vlc:id>
                <vlc:option>start-time=42</vlc:option>
                <vlc:option>stop-time=45</vlc:option>
                <vlc:option>no-audio</vlc:option>
                <vlc:option>some-option=100</vlc:option>
            </extension>
        </track>
        <track>
            <title>DVD seconds 49-55 unmuted (normal)</title>
            <location>dvd://d:\@1</location>
            <extension application="http://www.videolan.org/vlc/playlist/0">
                <vlc:id>4</vlc:id>
                <vlc:option>start-time=49</vlc:option>
                <vlc:option>stop-time=55</vlc:option>
            </extension>
        </track>
    </trackList>
 </playlist>

xref:

• Description for m3u in VLC forums
• Description for XSPF in VLC wiki

Which feature or functionality are you missing in Audacious?
What makes it beneficial for users?

Additional information

Willing to do a PR if maintainers willing to accept.