Add configurable parameters to changelog

[nunofgs]

Allows calling uphold-scripts changelog and manually specify the tag and release name.

Note that the argument parsing isn't perfect (neither was the original script). There's multiple ways to escape it but I feel that it's good enough for our usual use cases.

[rplopes]

The format -future-release <version> -future-release-tag v<version> was chosen because at that time, it seemed to be the convention for changelogs on our projects. Now, it seems to be -future-release v<version> -future-release-tag v<version>, which can be simplified to -future-release v<version>.

WDYT about changing the behaviour to start outputting in that format? It would probably simplify many projects' setups, without any flag. It would be a breaking change in this library, but as long as we follow semver, it looks like a pretty reasonable one.