Added support for adding custom parameters to icecast.xml

[LucaTNT]

I needed to set some custom mount options, so I added the ICECAST_EXTRA_PARAMETERS environment variable to allow users to specify custom xml to be added at the end of icecast.xml.

It works fine in my testing, but of course the user is free to enter invalid xml which would cause Icecast not to start. To be used with caution I guess 😃

[simonepozzoli]

In my opinion the best thing would be to directly mount a local icecast.xml file inside the container to override the settings. In this way you have full control over your settings and no other logic needs to be added to this repo.

[LucaTNT]

That's right, but IMHO it's handy to have both the ease of setup that docker-entrypoint.sh provides, as well as the flexibility of adding a couple parameters as raw XML.