Add Docker support

[cesarga]

I've just added a Dockerfile to build the image and updated the README.md with the usage information.

You can try the image I have already uploaded to Docker hub:

docker run --rm cesarga/fierce --domain google.com --subdomains accounts admin ads

[coveralls]

Coverage remained the same at 60.246% when pulling 9bd705d on cesarga:master into bfe3409 on mschwager:master.

[coveralls]

Coverage remained the same at 60.246% when pulling 9bd705d on cesarga:master into bfe3409 on mschwager:master.

[mschwager]

Hey @cesarga,

Thanks for opening a PR. I'd like to avoid supporting various packaging mechanisms in this repository (e.g. Docker images, .deb, .rpm, etc). This helps keep the delivery mechanism, and all the rabbit holes involved with that, separate from the code logic. However, I'd definitely encourage you to create a repository for a Fierce Docker image 👍

Thanks again!