Refactor internal logic and code duplication

[AndreLaranjeira]

There is a lot of code that could be refactored into private function implementations and logical representations that could be improved for readability.

Task list:

• Create a private function to encompass all 5 basic logging functions (they are the same aside from tag and context);
• Create a function to lock or unlock the mutex if thread-safety is enabled (this is used a lot in the code);
• Create boolean internal variables to replace checking if a pointer is null;
• Refactor internal time format representation to utilize the struct created instead of a simple string;
• Refactor color_background and color_text functions to use the enum representation of color. This should reduce the amount of cases in the switch;
• Create function that receives a MessageCategory and returns msg_type text. Use this to avoid hardcoded msg_type strings;
• Create set_time_format function that receives a TimeFormat pointer. Deprecate the other existing one;