Feature request/suggestion: Customizable prefix

I've just discovered this library, and I like it quite a lot, especially the lightweight header file design is really great! I was wondering if you'd consider making the prefix somewhat customizable? Concretely, in my application I don't need thread name and uptime, so it would be nice if it was possible to disable individual elements of the prefix.

So for example, having boolean flags in the options passed to `loguru::init` to turn each type of prefix on/off.

I could use `RAW_LOG_F`, but then I'd lose all the other prefixes - I'd still like to have date, time file name etc.