Added flatten and flatten(x) functions

Fix #415.

tests/all.test

@@ -846,3 +846,15 @@ map([1,2][0:.])
 [.[]|nulls]
 [1,2,"foo",[],[3,[]],{},true,false,null]
 [null]
+
+flatten
+[0, [1], [[2]], [[[3]]]]
+[0, 1, 2, 3]
+
+flatten(2)
+[0, [1], [[2]], [[[3]]]]
+[0, 1, 2, [3]]
+
+flatten(2)
+[0, [1, [2]], [1, [[3], 2]]]
+[0, 1, 2, 1, [3], 2]