Unable to run it under a proxy server

Running `textlint --rule textlint-rule-no-dead-link **/*.md` on the proxy server results in an error.

The environment variables `HTTP_PROXY` and `HTTPS_PROXY` are set, but I guess they are not working effectively because node-fetch does not support them.

https://github.com/textlint-rule/textlint-rule-no-dead-link/blob/master/src/no-dead-link.js#L2

