Support configurable Git tag format

[goulvenclech]

Introduce two configuration options, tag-format and short-tag-format, to let users customise the format of Git tags generated by Sampo. Both options must support interpolation of {package-name} and {version}.

Default configuration:

[git]
tag-format = "{package-name}-v{version}"
short-tag-format = "v{version}"

Open question: handling tag collisions
What is the best approach if tag-format does not include {package-name} and multiple packages might generate the exact same tag for a given version? Should Sampo prevent this at config-time, warn, or leave it to the user?

[rafaeelaudibert]

It must be during config time, correct? I don't see how we would warn later on, since after #191 you're NOT tagging if we find a matching tag already.

My theory is that we could prevent that with an error during initialization if we detect that we're in a multirepo situation + {package_name} isn't set on tag-format. Could even do the same for short-tag and block that from being set if we detect we're in a multirepo (separate issue)