I was unable to find a plugin that would allow me to play custom streams, so after days of learning Python, with help of shopgirl284, Mikedm139 and examples found on this forum, I created my first Plex plugin; I present You Plex plugin that plays network streams (a.k.a. IPTV) from a M3U playlist:
https://github.com/Cigaras/IPTV.bundle
Also available at Unsupported AppStore.
Included sample playlist is provided by MOCAICO with over 4000 channels, but it might be outdated and too big for some clients to handle, it is advised to trim it down to a few hundred channels, please read post #287 for more info. Also sergrusur started a new Spanish forum for IPTV related stuff, it takes some time and patience to register and change language to English there but it is doable, and it has many IPTV streams, check post #217 and further for more information. Also was192 has a whole blog dedicated to IPTV channels lists, check it out, and user isawred2 shared very nice playlist, head to post #275 while it is still available. You can find more playlists scattered along this thread (like posts #388, #399, #407, #417, #457, #500, #501 and so on) or just google for them.
At the moment plugin supports HTTP and RTSP streaming protocols, RTMP also works with some limitations, and MMS did not work for me on any device, some users report that it works in Android and Plex Web. Please keep in mind that this plugin does not actually play videos by itself, it only passes links to Plex in a format understandable to Plex and the playback is done by Plex client, and it is a known fact that Plex Media Server does not transcode live streams so you might need a third party transcoder like VODServer if client is unable to transcode streams by itself, please read post #97 (especially Apple TV owners) and compatibility section of readme file for more information.
One more thing, Plex Home Theater, the re-branded Plex Media Center, does not play live streams at all, in any way known to me, so if You want to use this plugin on PC or Mac, You need the old version of Plex Media Center; please read readme file for further indormation.
If plugin does not work of you don't like it or don't like Plex, but still want to watch IPTV on your TV or other device, You can check post #194 for alternative solutions, but in any way please consider making a small donation if I was of any help for You.
And instead of summary, if someone of more experienced developers could spare a few minutes to test and review my plugin I would be more than grateful, please help me perfect it to be suitable for the App Store.