Add a trivial callback sink

[maghorbani]

I needed such a functionality, to receive logs in a callback function, so I added this sink to the library.
The new sink, will call a custom callback, each time something is logged.
The code was initially cloned from basic_file_sink.h.
Required test, example and readme sample, added.

[gabime]

Thanks @maghorbani . I am not sure. Whats exactly the use case here? This can be achieved already by implementing custom sink just as easily.

[maghorbani]

Well I needed it to be like this, so my custom sink would not be an additional patch to spdlog in every single project of mine. I just thought that, it is not harmful to have a general purpose sink.
Anyway merging it or not, that's your decision, but I will keep my fork sync with the main repo and use it in my projects that use spdlog. I saw an issue or two, asking for the same kind of sink.

[gabime]

If there a popular demand for such think, I don't mind adding this. Which issues mention this?
As a side note, I would make it simpler for use by getting rid of the struct and just providing 2 constructor overloads that accept the lambdas.
Also note that the conversion to string in the second lambda is not cheap since string allocates. Better to reuse a private member string.

[maghorbani]

here are some that I found, searching callback keyword in issues:
2469, 1328, 1016
or 863, they did not know how to implement a new sink, but such a callback sink would meet their need
and an old one (i just saw it) 205, they implemented such a callback, but it was not general enough to be merged.

I'm not sure if it worth it to be merged, if so, please let me know, and I will improve the new sink to meet your reasonable advice:

As a side note, I would make it simpler for use by getting rid of the struct and just providing 2 constructor overloads that accept the lambdas.
Also note that the conversion to string in the second lambda is not cheap since string allocates. Better to reuse a private member string.

[gabime]

I'm not sure if it worth it to be merged

Lets start with a very basic, single lambda callback (the one with the the log_msg arg).
This I think should cover most of the cases.
If more functionality needed, it can be added later.

Thanks!

[gabime]

Thanks @maghorbani . Merged.