Add automatic behaviors

[asasine]

• Download puzzle input to a cache location for the day being executed #5
• Add CLI that can execute any day #3
• Add new day and new year scaffolding #4

[asasine]

So far, it seems that puzzles always yield numeric results, and part 2 alwals needs a slightly different way to parse the input data, so a standard form for the solution functions seems possible:

fn solution(input: &str) -> usize {
    // --snip--
}

[asasine]

I'm thinking I'll steal from the unix philosophy here and lean into each day being its own binary. This is what I did for #3 and it means I can get some potential benefits:

1. Each binary can be as flexible as necessary for that day's given puzzle.
2. The runner can work with the simple assumption that the day's binary writes the solutions to separate lines on stdout, one line per part. If desired, this assumption can also lend itself to the runner submitting the result automatically.
3. Any informational logs can be written to stderr ad nauseam.
4. The runner can feed puzzle input via stdin, meaning puzzle input can be lazily fetched and cached by the runner the first time a puzzle is executed.