Added a optional parameter to define a fixed tag for changelog generation

[turleypol]

When doing with your action an automatic "nightly" release with a fixed tag name it currently only generates the changelog between current and old tag. For this usecase its better to have an additional parameter to define a fixed tag as changelog reference.

[marvinpinto]

I think this might be related to #68. I want to think about what the API will look like before adding more options. I'll leave this open for now.

[kian2attari]

+1 on this, it would be really valuable to be able to specify the previous tag the changelog should be generated from

[kian2attari]

This would likely also be valuable for #16 and #212

[okias]

having a possibility to define something like HEAD~1 would be very nice, when is repository often rebased against some upstream project, where the commit diff can be huge.

[stefanotorresi]

@marvinpinto any chances to see this picked up again? What API would you like instead? I could maybe work on something.