Add 256 colors support

Themes can now use color names from the xterm color name list [1].

1: https://jonasjacek.github.io/colors/

src/config/color.h

@@ -0,0 +1,12 @@
+#ifndef _COLOR_H_
+#define _COLOR_H_
+
+/* to access color names */
+#define COLOR_NAME_SIZE 256
+extern const char *color_names[];
+
+/* to add or clear cache */
+int color_pair_cache_get(const char *pair_name);
+void color_pair_cache_reset(void);
+
+#endif