Add base sha option for changelog generation and release

[iliapolo]

PyCI changelog generation is performed with a source sha and a target sha.

However, the source (i.e base) sha is not configurable. It always uses the sha of the latest release prior to the target sha.

This mechanism is problematic for existing projects that haven't been producing github releases. If and when these projects choose to work with PyCI, their very first release will include ALL commits in the project. This, apart from being a very slow process, is a poor experience.

To support this scenario, we can add a --changelog-base option for overriding the default behavior. This way users can specify the sha they want on initial release, after which they can . remove that specification and let PyCI do the work.

[iliapolo]

This issue is part of release 0.6.0