Optionally use external spdlog library

[swltr]

As mentioned in #7, it would be nice to allow external dependencies to be used.
This helps e.g. if you already have spdlog as a dependency in your CMake build.
With this small change, it is possible to turn off auto-detecting spdlog.
Leaving the target_link_libraries(spdlog_setup ...) calls as they are, these then simply use an existing spdlog library.
You need to add that library somewhere else in your build, then.
In my case, I have a line like

add_subdirectory(libs/cmake/spdlog-1.3.1)

in my top-level CMakeLists.txt.

[codecov]

Welcome to Codecov 🎉

Once you merge this PR into your default branch, you're all set! Codecov will compare coverage reports and display results in all future pull requests.

Thanks for integrating Codecov - We've got you covered ☂️

[swltr]

@guangie88 What do you think about this?

[swltr]

@guangie88 Any thoughts on this?