`security` -> `severity`

[jackielii]

I realised I was reading it wrong after searching severity but couldn't find one, then realised it's called security. But I think we should definitely fix it.

It should be a backward compatible one by just adding the new option and assign old option to the new one. Then search & replace the docs.

[jackielii]

Same thing in https://github.com/iamcco/coc-diagnostic

[iamcco]

Thanks for pointing out