Add option to force color output without TTY

This adds a new "color_mode" enum that can be used to control the color
code output behavior of sinks with color support. It can be one of three
values: always, automatic and never.

Remove unnecessary semicolons (-Wextra-semi)

Add a CMake option to use a header-only external fmt

Update README.md

In order to use basic_logger_mt an additional include is required.

Fix unexpected log macro expansion.

Namespace qualify stat functions

Add spdlog::initialize_logger

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