Consider adding the trailing separator as optional for optional values

When defining a route like this:

```
/foo/{param}
```

with a default value for param, you can omit the param and Symfony will match a URL like `/foo`. But it won't match `/foo/`.

Would it make sense to allow it? What are the consequences on BC?

Or do we need to introduce a special syntax for optional parameters:

```
/foo/{?param}

/foo{/?param}
```

see fabpot/Silex#485
