Add --skip-existing option to ndenv install.

[mshibanami]

Currently, I use this command to install the latest node version.
% ndenv install $(ndenv install -l | grep -v - | tail -1)

It works good. but I get this message if the current version is the latest one.

ndenv: /Users/mshibanami/.anyenv/envs/ndenv/versions/v4.2.1 already exists
continue with installation? (y/N)

It's a bit annoying.

rbenv solves the same issue with --skip-exisiting option.
So I also need it for ndenv install.

[riywo]

Done. #5