rcfile: allow specifying a bright background color (with prefix "light")

If the terminal is capable of more than eight colors, specifying
indexes 8 to 15 works fine for getting a light background color.

src/rcfile.c

@@ -1031,12 +1031,14 @@ bool parse_combination(char *combostr, short *fg, short *bg, int *attributes)
 
 	if (comma != NULL) {
 		*bg = color_to_short(comma + 1, &vivid, &thick);
-		if (vivid) {
+		if (vivid && thick) {
 			jot_error(N_("A background color cannot be bright"));
 			return FALSE;
 		}
 		if (*bg == BAD_COLOR)
 			return FALSE;
+		if (vivid && COLORS > 8)
+			*bg += 8;
 		*comma = '\0';
 	} else
 		*bg = USE_THE_DEFAULT;

syntax/nanorc.nanorc

@@ -21,7 +21,7 @@ color green "^[[:space:]]*((un)?(bind|set)|include|syntax|header|magic|comment|f
 color brightmagenta "([[:space:]]|(start|end)=)".+"([[:space:]]|$)"
 
 # Colors
-color yellow "^[[:space:]]*i?color[[:space:]]*(bright|light)?(white|black|red|blue|green|yellow|magenta|cyan|normal)?(,(white|black|red|blue|green|yellow|magenta|cyan|normal))?\>"
+color yellow "^[[:space:]]*i?color[[:space:]]*(bright|light)?(white|black|red|blue|green|yellow|magenta|cyan|normal)?(,(light)?(white|black|red|blue|green|yellow|magenta|cyan|normal))?\>"
 color magenta "^[[:space:]]*i?color\>" "\<(start|end)="
 
 # Comments