Add tasks for all the new languages

Task/String-append/EchoLisp/string-append.echolisp

@@ -0,0 +1,9 @@
+;; Solution from Common Lisp and Racket
+(define-syntax-rule (set-append! str tail)
+   (set! str (string-append str tail)))
+
+(define name "Albert") → name
+
+(set-append! name " de Jeumont-Schneidre")
+name
+   → "Albert de Jeumont-Schneidre"