core: add an argument to keep colors in no ui mode

[romainreignier]

Following my https://github.com/xqms/rosmon/issues/185#issuecomment-2136813205, here is a proposition about adding a mode to disable UI but keep the colors in the logs (or more precisely, adding colors based on the severity).

As explained in the issue, I have chosen to not detect if run in a TTY to enable colors automatically because in my use case, rosmon is started as a sub-process, hence not directly in a TTY.

I am not sure about the name of the argument and its short version, feel free to propose other names.

I am not a big fan of adding yet another global variable in main.cpp but I kept the same solution than when I have added the flush to stdout flag.