Add ansi256 and bgAnsi256 to TypeScript declarations (#368)

index.d.ts

@@ -213,6 +213,11 @@ declare namespace chalk {
 		*/
 		hwb(hue: number, whiteness: number, blackness: number): Chalk;
 
+		/**
+		Use a [8-bit unsigned number](https://en.wikipedia.org/wiki/ANSI_escape_code#8-bit) to set text color.
+		*/
+		ansi256(index: number): Chalk;
+
 		/**
 		Use HEX value to set background color.
 
@@ -261,6 +266,11 @@ declare namespace chalk {
 		*/
 		bgHwb(hue: number, whiteness: number, blackness: number): Chalk;
 
+		/**
+		Use a [8-bit unsigned number](https://en.wikipedia.org/wiki/ANSI_escape_code#8-bit) to set background color.
+		*/
+		bgAnsi256(index: number): Chalk;
+
 		/**
 		Modifier: Resets the current color chain.
 		*/