add --dir option. chdir before exec

add --dir option to start_server.
It is useful in combination with proclet and carton. like this

    $ start_server --port port --dir /path/to -- carton exec -- plackup app.psgi

as same as this. but --dir is more simple.

    $ start_server --port port -- sh -c 'cd /path/to && carton exec'