Sub path support

[yosiat]

Hi,

I am using this middleware and I am mounting my api server under specific sub-path (using echo.Group("/subpath")), when using the subpath I am getting an error of no matching operation was found.

Without the sub-path it works.

Looks like it's happening because we are constructing a new router with gorillamux and not re-using existing router.

Any idea on how to overcome this issue?

[GreyXor]

I also try to have my api under /api but then all my other endpoint doesn't pass the validation because /foo for example, is not in my api definition.

[GreyXor]

hello @yosiat, would you try my PR and tell me if that fit your needs ?