add chaining / mounting routing tables

## Long story short
```python
module = web.RouteTableDef()

@module.get('/status')
async def status(handler):
return web.Response(text="I am fine.")

root = web.RouteTableDef()
root.mount("/api/v1", api_module)
```
Now we got `/api/v1/status` as a route.

## Implementation
If there is interest in having this feature I am willing to look into implementing it and submitting a pull request. I just would like to first clarify API definitions.