v0.0.2

增加两个便捷函数，让标准库map更好用些。

mapex.Keys()
m := make(map[string]string)
m["a"] = "1"
m["b"] = "2"
m["c"] = "3"
get := mapex.Keys(m)// 返回map的所有key
mapex.Values()
m := make(map[string]string)
m["a"] = "1"
m["b"] = "2"
m["c"] = "3"
get := mapex.Values(m)