Add tasks for all the new languages

Task/Sorting-algorithms-Merge-sort/Crystal/sorting-algorithms-merge-sort.crystal

@@ -0,0 +1,18 @@
+def merge_sort(a : Array(Int32)) : Array(Int32)
+  return a if a.size <= 1
+  m = a.size / 2
+  lt = merge_sort(a[0 ... m])
+  rt = merge_sort(a[m .. -1])
+  return merge(lt, rt)
+end
+
+def merge(lt : Array(Int32), rt : Array(Int32)) : Array(Int32)
+  result = Array(Int32).new
+  until lt.empty? || rt.empty?
+    result << (lt.first < rt.first ? lt.shift : rt.shift)
+  end
+  return result + lt + rt
+end
+
+a = [7, 6, 5, 9, 8, 4, 3, 1, 2, 0]
+puts merge_sort(a) # => [0, 1, 2, 3, 4, 5, 6, 7, 8, 9]