add configuration for ternjs queries

[andykais]

It would be nice if we could configure the completions we get back from ternjs like deoplete-ternjs offers: https://github.com/carlitux/deoplete-ternjs#vim-configuration-example. The default ternjs completions can be a tad too noisy. E.g. including keywords like const and function.

I see the options are already being set in the python file https://github.com/ncm2/ncm2-tern/blob/master/pythonx/ncm2_tern.py#L66. If I were pointed in the right direction, I could open a PR for this.

And of course, thank you for the awesome work on ncm2!