Add 'github.com/gorilla/websocket' to 'go.mod' file

[ShoshinNikita]

First, thank you for the great tool! It really helps to visualize and analyze runtime stats

I noticed that package websocket is a copy of github.com/gorilla/websocket. I would suggest to remove it from the repo and add to go.mod file. Even if there are some changes, it's better to create a fork. What do you say?

[arl]

When I started the project, statsviz http handlers were installed with an init, _ import like you'd do with net/http/pprof handlers. I wanted the library to be self-contained that's why I included websocket in the codebase.
I think you're right, now that's not useful anymore and it can even be counterproductive for projects which already import websocket as they would have a duplicate dependency.
Will add this to go.mod.

Thanks for reporting.