Add true color detection

src/utils.cpp

@@ -60,6 +60,12 @@ void term::progress(int prefixlen,
     *_out << ' ' << std::setw(3) << perc << '%';
 }
 
+bool term::has_truecolor() const {
+    char* tc = getenv("COLORTERM");
+    std::string colorterm {tc ? tc : ""};
+    return colorterm.size() > 0 && (colorterm == "truecolor" || colorterm == "24bit");
+}
+
 // ------------------------------------------------------------------------
 
 cursor_hider::cursor_hider(std::ostream* out) : _out {out} {