Feature request: Disable diagnostics

### Environment

* Elixir & Erlang versions (elixir --version):
* Elixir Language Server version:
* Operating system:
* Editor or IDE name (e.g. Emacs/VSCode):
* Editor Plugin/LSP Client name and version:

### Current behavior

The plugin underlines warnings that you usually get from the compiler, such as the following.

```
variable "stream" is unused (if the variable is not meant to be used, prefix it with an underscore)
```

I have a codebase that has a lot of these warnings, and it's supper annoying to try and program in a codebase that's full of these lines.

### Expected behavior

Is it possible to add an option to disable these squiggly lines? Other language extensions seem to have that option: https://social.msdn.microsoft.com/Forums/sqlserver/en-US/b4278905-b34a-4396-945d-d7d661a29b21/how-to-hide-warning-scribblies-in-vs-code

Thanks for the plugin and all the hard work!