Forked and maintaining - Updated to `go mod` and updated `xtermjs => 4.17`

[ghthor]

I'm leaving this PR here as an option for anyone looking to use gotty. I'm maintaining it in at the current feature set while I've updated to use go mod and I've updated the xtermjs library version to the most current.

Thanks to yudai for making an excellent piece of software!

[ghthor]

It also looks like this Fork[1] is actively maintained as well. I didn't find it because it wasn't forked proper from this repository and that's what I was looking at to try and find the actively maintained fork.

[1] https://github.com/sorenisanerd/gotty