vim-lsp support (formatting not working)

Hi everyone, this is great - I've been trying to get this working with https://github.com/prabirshrestha/vim-lsp but have trouble running the formatting command:

```
"https://github.com/jaredly/reason-language-server/releases/latest/download/bin.exe
if executable('reason-language-server')
au User lsp_setup call lsp#register_server({
\ 'name': 'reason-language-server',
\ 'cmd': {server_info->[&shell, &shellcmdflag, 'reason-language-server --stdio']},
\ 'root_uri':{server_info->lsp#utils#path_to_uri(lsp#utils#find_nearest_parent_file_directory(lsp#utils#get_buffer_path(), 'bsconfig.json'))},
\ 'whitelist': ['reason'],
\ })
endif
```

This seems to work for [ocaml](https://github.com/prabirshrestha/vim-lsp/wiki/Servers-OCaml---Reason):

```
if executable('ocaml-language-server')
au User lsp_setup call lsp#register_server({
\ 'name': 'ocaml-language-server',
\ 'cmd': {server_info->[&shell, &shellcmdflag, 'ocaml-language-server --stdio']},
\ 'whitelist': ['reason', 'ocaml'],
\ })
endif
```

Thanks!

Start with the vim-lsp configuration in the issue, especially the reason-language-server --stdio command and the formatting command that fails. Compare it with the working ocaml-language-server configuration and investigate how reason-language-server handles formatting. Done means formatting works through vim-lsp for Reason files.

Written by the indexing model from the issue text.