Year to date support

[nandosobral03]

Another potential proposal for to address #33

With this change you can now do.

% ./gh-skyline --user nandosobral03 --year-to-date
% ./gh-skyline --user nandosobral03 --year-to-date --until 2025-01-01

And with this generate the skyline for commits from the past year (by default matching with what is shown by default on your github profile)

I'm not 100% sure about the CLI UX but wrote this mainly as something I wanted to use and to propose an alternative to the period based solution. It does have to it's favor that we don't need to worry about varying widths since it's the same number of weeks as the yearly solution