Add example for accumulate

Author: erichaney

examples/accumulate.janet

@@ -0,0 +1,8 @@
+(reduce + 0 [1 2 3 4]) #-> 10
+(accumulate + 0 [1 2 3 4]) # -> @[1 3 6 10]
+
+(reduce + 0 []) # -> 0
+(accumulate + 0 []) # -> @[]
+
+(reduce string "" ["J" "a" "n" "e" "t"]) # -> "Janet"
+(accumulate string "" ["J" "a" "n" "e" "t"]) # -> @["J" "Ja" "Jan" "Jane" "Janet"]