Feature request: allow token from environment variable and add -y switch for avoiding prompt

[tjaneczko]

We'd like to use gh-release through a CI/CD system (Jenkins). We can include a GitHub token as an environment variable, and would like to use that as we have some projects on github.com and some projects on an enterprise github (the current ghauth supports one or the other, not both). Adding support for a GH_RELEASE_GITHUB_TOKEN environment variable would be perfect. Additionally, we would want to bypass the yes/no prompt for actually performing the publish, so something like a -y switch would be perfect.

If this all sounds good, you can assign this to me, as I'll probably end up working on this soon.

[ungoldman]

These feature requests both sound totally reasonable!

Please feel free to tackle these issues.