Allow configuring the behavior of the release tool using a YAML file

[nicwortel]

At the very least, this should allow users to configure the "release steps" that should be executed when releasing a new version.

The release tool should look for a file in the current working directory (something like .release-tool.yml) and read the contents to apply the configuration.

Example .release-tool.yml, with default values:

versioning-scheme: semantic

vcs:
  git:
    tag-prefix: v
    remote: origin

post-release-actions:
  github-release: ~

The configured values should be used when compiling the service container.

See https://github.com/Behat/Behat, https://github.com/sensiolabs-de/deptrac for examples.

• design a good and scalable configuration format
• use symfony/yaml to read the file

Probably requires #7