Implemented strikethrough text

Render.go

@@ -257,11 +257,12 @@ func (r *renderer) writeText(markdown string) {
 		codeStart     = -1
 		emStart       = -1
 		strongStart   = -1
+		strikeStart   = -1
 		parentheses   = 0
 	)
 
 	for {
-		i := strings.IndexAny(markdown[searchStart:], "[]()`*_")
+		i := strings.IndexAny(markdown[searchStart:], "[]()`*_~")
 
 		if i == -1 {
 			r.WriteString(html.EscapeString(markdown[tokenStart:]))
@@ -342,6 +343,23 @@ func (r *renderer) writeText(markdown string) {
 				tokenStart = i
 				emStart = i + 1
 			}
+
+		case '~':
+			if i+1 >= len(markdown) || markdown[i+1] != '~' {
+				continue
+			}
+
+			if strikeStart != -1 {
+				r.WriteString("<del>")
+				r.WriteString(html.EscapeString(markdown[strikeStart:i]))
+				r.WriteString("</del>")
+				strikeStart = -1
+				tokenStart = i + 2
+			} else {
+				r.WriteString(html.EscapeString(markdown[tokenStart:i]))
+				tokenStart = i
+				strikeStart = i + 2
+			}
 		}
 	}
 }