Feat(serve) : Add flatten flag to serve command
Feat(serve) : Add flatten flag to serve command

Fix #2078

The goal is to be able to serve a swagger spec containing external
$ref.

* Added a `Flatten` flag to `ServeCmd`
* Flatten spec if flag is present

Signed-off-by: Sébastien Le Gall <10c8c65d21546f2c1addad6f3fec743aa1c8bafb@meetic-corp.com>

go-swagger/go-swagger,go-swagger/go-swagger