Use Flask routing to allow for variables in URL

This allows for variable URLs with no change to the current API.

```
@sockets.route('<name>/receive'):
def receive(ws, name):
    ...
```

I saw that there was already a pull request for this here: https://github.com/kennethreitz/flask-sockets/pull/7, but I think the solution proposed in this PR is cleaner, with no breaking changes. Based on the other pull request, it looks like a lot of people want this feature.

@kennethreitz what do you think?