Default kind_labels to CompletionItemKind

[wbthomason]

Per discussion with @Shatur95 in #49

[Shatur]

Does anyone need to have kind_labels empty? If not, then we can read CompletionItemKind if kind_labels is not set.
Or, alternatively, it could be an additional boolean option.

[dhruvmanila]

You can do this: if the kind_labels field is nil then default to CompletionItemKind otherwise use that and if the user does not want any labels then they can enter an empty table like kind_labels = {}.

Thinking more about it, I won't prefer to use the default. For CompletionItemKind, I have set it up like <icon> <kind>:

And for lsp-status plugin which I'm using in the statusline, I have set it up like <icon>

I think most people would prefer the verbose version for the completion menu and icon only version for the statusline.