Binding for URL Params

Is there a way to validate URL params?

e.g. 

```go
r.GET("/:foo/*bar", myFunc)
```

How can I validate foo and bar?

It would be nice if c.Bind() method could fall back (query params, form params, url params) - maybe introducing a 'url' tag:

```go
type MyParams struct {
    Foo string `url:"foo"`
    Bar string `url:"bar`
    Download bool `form:"dl"`
}
```
