Allow Git tags to generate Docker tagged images

[dduportal]

This PR closes #54 .

It allows triggering a Travis CI build by creating a git tag or a github release,
that results in a a tagged Docker image.

Please note the following elements:

• The tag latest will be kept, containing the image built from the HEAD of the branch `master as it was before
• Git tag can happen on any branch, not only master
• The variable TRAVIS_BRANCH is used to pass the tag being built, as per https://docs.travis-ci.com/user/environment-variables#default-environment-variables . This documentation specifies that this variables contains the git tag which triggered the build (e.g. the same value as $TRAVIS_TAG), or the branch if it is "push-triggered" build (or the target branch if it is a PR).
• Tags and branch names with a / are supported: there is a search and replace rule which turns / to - for the resulting Docker tag, as the slash is not a valid character for a Docker tag.

[dduportal]

For demonstration purpose, a "dummy" tag named test-release had been created:

• A Travis-CI build had been triggered: https://travis-ci.org/asciidoctor/docker-asciidoctor/builds/648137094
• Which triggered a DockerHub build, after tests passed: https://hub.docker.com/repository/registry-1.docker.io/asciidoctor/docker-asciidoctor/builds/a9d799d3-dc12-458d-8bcf-0abc226e8082
• Result was a tagged Docker image.

Please note that, even if the README.md is generated for a tagged release, the only updated of the README page in the Dockerhub comes from the file README.md on the master branch only.

[dduportal]

👋 @mojavelinux if it is ok , I'll let you the honor to draft the first 1.0.0 after merging this PR.

I'll take care of cleaning up the "dummy" tag in bot the repo and DockerHub (and I'll start working on the renaming in #79)

[mojavelinux]

Unless you really need me to do it, I think it would be more appropriate if you cut the release. I'm just here to make sure you have what you need so it goes smoothly ;)

[dduportal]

Sounds good then: I need a review + LGTM then.

Thanks Dan!

[mojavelinux]

LGTM!