Add built-in ANSI escape code shortcut combinations and custom terminal output

[alexwkleung]

Plan on adding built-in ANSI escape code shortcut combinations such as bold and underlined foreground/background colours (8/16 colours; not 256 colours). It would be nice to have the option to have shortcuts for these combinations so the syntax doesn't become verbose and tedious.

Here are some examples of shortcut combinations:

#include "termstylist.h"
#include <iostream>

int main() {
  std::cout << termstylist::Bold_FG_Cyan << "Bold Cyan!" << termstylist::RESET_Default << '\n';

  return 0;
}

#include "termstylist.h"
#include <iostream>

int main() {
   std::cout << termstylist::Bold_Underline_FG_Magenta <<  "Bold Underline Magenta!" << termstylist::RESET_Default << '\n';

   return 0;
}

An example of what the shortcut combinations would replace (as shortcut options, to eliminate verbose syntax):

#include "termstylist.h"
#include <iostream>

int main() {
    std::cout << termstylist::ATTRS_Bold << termstylist::ATTRS_Underline << termstylist::FG_Yellow << "Bold Underline Yellow!" << termstylist::RESET_Default << '\n';

   return 0;
}

Also, custom terminal output such as coloured loading/percent bars would also be good to have for niche versatility of the library.

Here's an example of what that could look like (syntax wise):

#include "termstylist.h"
#include <iostream> 

int main() {
   std::cout << "This is a loading bar!" << '\n' << termstylist::GreenLoadingBar << std::flush;

   return 0;
}

If any of these features are going to be added, make sure to add unit tests for them whenever possible.