Added def-behave->, def-behave->>, and documentation

def-behave-> and def-behave->>

```clojure
user> (r/def-behave-> b a inc)
#'user/b
user> @b
2
user> (swap! a dec)
0
user> @b
1
user> (r/def-behave->> b a (str "a = "))
#'user/b
user> @b
"a = 0"
```