Allow configuration of socket.io transports

[pavelkucera]

Hi. I would like to use Artillery to test a socket.io application which doesn't support long polling, which is not possible with current implementation of Artillery. Without specifying that long polling isn't available socket.io tries connecting via long polling and fails.

This pull request allows using following structure in config:

config:
    socketio:
        transports: ["websocket"]

To tell socket.io-client that it should connect only using websockets. When the section is not defined, default values are used.

I'll be happy to adjust the code if necessary.

[hassy]

Looks good 👍

[pavelkucera]

Thank you!

[pavelkucera]

• In the original comment the configuration example was wrong (I missed the socketio section). I corrected it here.