[Feature]: Generate for YTD/last 12 months timeframe

### Description

My 2024 has been a good year for me, contribution-wise, but 2024 isn't over! So, there is a gap at the end of my timline. It would be nice to generate my contributions for the last 12 months i.e. `gh skyline --ytd` maybe.

I guess the tricky part would be what is generated in the bottom right for the date 🤔 I suppose once you build something like this, there is not reason to not extend to any 12 month timeframe.

### Requirements

Not sure, I don't know much Go, haha

### Definition of Done

There is some option to generate a skyline for the last 12 months of contributions.

### Additional Notes

Probably a good first issue 🙃