add some optional fields to error notice context

[blackghost1987]

These optional context field are required by my project, so I added them to the config

[kyrylo]

This should be configurable on notice level (same with os, hostname). Ideally, os & hostname are figured automatically.

[blackghost1987]

Yeah I made "hostname" to be automatically set in my app with the "nix" crate, but it required some conditional compilation as it's not working on windows. I can add it though in another merge request, if you think it's useful. Maybe we can even find a way to do it on windows properly as well.
I'm not sure about the "os", but maybe we can get it from the cfg! macro with "target_os". I'm not sure it can return a string though, it looks like it's returning a boolean only, so maybe requires some "if ... else if ... else if" magic.

What do you mean about making the "environment" and the "component" configurable on the notice level? Is there any way to pass arguments for one specific notice now? If not how should we implement it? Maybe until such a feature is not ready, we can add these as "defaults" for the future arguments, just to get it working for now.