Color: Improve colorize() performance

- Improves "render" time by ~45-55% across relevant performance tests.
- Improves "list" performance test by ~20%.
- Improves "all" performance test by ~40%.

src/Color.h

@@ -61,6 +61,7 @@ public:
 
   std::string colorize (const std::string&);
   static std::string colorize (const std::string&, const std::string&);
+  void _colorize (std::string&, const std::string&);
   static std::string strip (const std::string&);
 
   bool nontrivial () const;