Introduce colored logging output for Unix & Windows platforms

* ConsoleOutput is the pubic interface (controls UnixConsoleOutput and
WindowsConsoleOutput). ConsoleOutputTest has been added for testing of
the interface.

* SDL2Logger: Introduce log output callback to use for the new colored
output feature.

* cmake/platform.cmake: Introduce NOM_PLATFORM_POSIX var; this gets set
when OSX or LINUX is detected, in order to ease the sharing of common
source files between the operating systems.

* Relocate SDL2LoggerTest from 'tests/src/system' to 'tests/src/core'.

Resolves Issue #16
Split off SDL2Logger dependencies to new top-level dir