feat: add ANSI color/style formatting

- at some log levels by defaults,
- can be used within format

tests/t-color.cpp

@@ -0,0 +1,45 @@
+#include <iostream>
+#include <limits>
+
+#include "../clutchlog/clutchlog.h"
+
+int main(/*const int argc, char* argv[]*/)
+{
+    using typo = clutchlog::fmt::typo;
+    using fg = clutchlog::fmt::fg;
+    using bg = clutchlog::fmt::bg;
+
+    clutchlog::fmt     none;
+    clutchlog::fmt      end(typo::reset);
+    clutchlog::fmt     note(typo::bold);
+    clutchlog::fmt     info(fg::green);
+    clutchlog::fmt  warning(fg::magenta, typo::bold);
+    clutchlog::fmt    error(fg::red,     typo::bold);
+    clutchlog::fmt critical(bg::red,     typo::underline, fg::black);
+
+    auto& log = clutchlog::logger();
+    log.threshold(clutchlog::level::info);
+
+    // Change a style.
+    log.style(clutchlog::level::quiet, error);
+    CLUTCHLOG(quiet,"Styles demo");
+
+    CLUTCHLOG(info,"Either using functions...");
+    std::cout << none("No style: ") << std::endl;
+    std::cout << note("NOTE: bold") << std::endl;
+    std::cout << info("INFO: green") << std::endl;
+
+    CLUTCHLOG(info,"... or tags.");
+    std::cout << warning  << "WARNING"    << end << ": bold magenta" << std::endl;
+    std::cout << error    << "ERROR"      << end << ": bold red" << std::endl;
+    std::cout << critical << "CRITICAL"   << end << ": underlined black over red background" << std::endl;
+
+    std::ostringstream format;
+    clutchlog::fmt discreet(clutchlog::fmt::fg::white);
+    format << "{level}: "
+        << discreet("{file}:")
+        << clutchlog::fmt(clutchlog::fmt::fg::yellow) << "{line}"
+        << clutchlog::fmt(clutchlog::fmt::typo::reset) << " {msg} ! " << std::endl;
+    log.format(format.str());
+    CLUTCHLOG(quiet,"After having inserted styles within a new format template");
+}