Feature request/suggestion: Customizable prefix

[lethal-guitar]

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.