Option for thread safe logging

[yalov]

I like this logging framework, it is header-only and tiny, could you make it also thread safe? (Optionally)

[martinmoene]

Thanks for your interest.

Do you have suggestions to make it easier to start on it?

[yalov]

The most simple way to enable feature for users that I know would be #define thread_safe 1 in the start of the clue.hpp, and by default the feature would be disabled.

No other suggestions from me, sorry.

[martinmoene]

Okay, thanks nonetheless.