Respect standard proxy header for hostname when generating M3U playlist

[dtruebin]

Fixes #610

First commit makes preparations, with no effective change, whereas the 2nd has the actual fix.
This is my first exposure to Go, so pardon me for not writing any tests 🥺

Manual testing done:

1. Default address — playlist entries point to the proper address 🟢

    $ curl \
    > 'http://localhost:8090/stream/Big%20Buck%20Bunny.m3u?link=dd8255ecdc7ca55fb0bbf81323d87062db1f6d1c&m3u'
    #EXTM3U
    #EXTINF:0,Big Buck Bunny.mp4
    #EXTVLCOPT:input-slave=http://localhost:8090/stream/Big%20Buck%20Bunny.en.srt?link=dd8255ecdc7ca55fb0bbf81323d87062db1f6d1c&index=1&play#http://localhost:8090/stream/poster.jpg?link=dd8255ecdc7ca55fb0bbf81323d87062db1f6d1c&index=3&play#
    http://localhost:8090/stream/Big%20Buck%20Bunny.mp4?link=dd8255ecdc7ca55fb0bbf81323d87062db1f6d1c&index=2&play
   

2. Pretend we are behind a reverse proxy — playlist entries point to the proper address 🟢

    $ curl \
    > -H 'X-Forwarded-Proto: https' \
    > -H 'X-Forwarded-Host: torr.example.org' \
    > 'http://localhost:8090/stream/Big%20Buck%20Bunny.m3u?link=dd8255ecdc7ca55fb0bbf81323d87062db1f6d1c&m3u'
    #EXTM3U
    #EXTINF:0,Big Buck Bunny.mp4
    #EXTVLCOPT:input-slave=https://torr.example.org/stream/Big%20Buck%20Bunny.en.srt?link=dd8255ecdc7ca55fb0bbf81323d87062db1f6d1c&index=1&play#https://torr.example.org/stream/poster.jpg?link=dd8255ecdc7ca55fb0bbf81323d87062db1f6d1c&index=3&play#
    https://torr.example.org/stream/Big%20Buck%20Bunny.mp4?link=dd8255ecdc7ca55fb0bbf81323d87062db1f6d1c&index=2&play
   

   For comparison, at 28f4f94 (current master) — playlist entries point to the proper schema 🟢 but wrong address 🔴

    $ curl \
    > -H 'X-Forwarded-Proto: https' \
    > -H 'X-Forwarded-Host: torr.example.org' \
    > 'http://localhost:8090/stream/Big%20Buck%20Bunny.m3u?link=dd8255ecdc7ca55fb0bbf81323d87062db1f6d1c&m3u'
    #EXTM3U
    #EXTINF:0,Big Buck Bunny.mp4
    #EXTVLCOPT:input-slave=https://localhost:8090/stream/Big%20Buck%20Bunny.en.srt?link=dd8255ecdc7ca55fb0bbf81323d87062db1f6d1c&index=1&play#https://localhost:8090/stream/poster.jpg?link=dd8255ecdc7ca55fb0bbf81323d87062db1f6d1c&index=3&play#
    https://localhost:8090/stream/Big%20Buck%20Bunny.mp4?link=dd8255ecdc7ca55fb0bbf81323d87062db1f6d1c&index=2&play