Add systemd sink.

Expose should_do_colors_ in ansicolor_sink.h

Add tweak support for user short level names

Fix typo in Android logger

Add CMake option for conditional installation

Remove unnecessary log function

Allow custom formatter in sink's ctor

Allow the inclusion of an external fmtlib as as sub_directory

Shadowed parameter

However, not in bundled fmt. Corrects compiling with basic logger with header only and -Werror.

CMAKE_BINARY_DIR -> CMAKE_CURRENT_BINARY_DIR

Pwm1234/rotate on open

and improve test for rotate_on_open=true

Add method to filesinks to return filename

Disable building benchmarks by default

Building the benchmarks requires google-benchmark to be installed which is
not the case for most of the build systems. The option now has a tiny
hint what the requirement is and where to get it.

closes #975