[Feature]: Option for slanted edges

### Description

Currently, the base of the output model has vertically straight edges. Although this looks fine by itself, it doesn't match models previously created/printed from the old GH skyline experience (https://skyline.github.com/). In said solution, the edges were slightly slanted as to make the bottom of the base slightly larger than the top.

Having a command line flag to generate the model with these slanted edges would more easily let users 3D-print the skyline matching their previous ones from the old website, thus eliminating the need to re-print previous years for consistency across models.

### Requirements

- Any way for the user to specify that a slanted base is wanted.

### Definition of Done

- [ ] A command line flag or option (see additional notes) letting the user choose to have a slanted base
- [ ] Updated command line flag/option documentation

### Additional Notes

There's two approaches I can see to this:
- letting the user pass a flag (for example `-s` for *slanted base*). When this flag is passed, the command will generate a model with a slanted base.
- Introducing an option for "base type" (or something similar), having multiple choices for how the user wishes the base to be generated. This is probably overkill for this minor change, but may be extended in the future if more model types are to be added.
- Examples: `gh skyline --base-type flat` and `gh skyline --base-type slanted` will produce models with the current base design or a slanted base design, respectively. Defaults to `flat` if omitted.