Advent of Code helper mode.

 - Global minor mode autodetecting solution directories

 - Display year/day/cookie in the mode line

 - Manage the AoC session cookie

 - Optionally persist the cookie in =.authinfo=

 - Create a new AoC day (with boilerplate files)

 - Browse existing day solutions

 - Open the current problem using eww

 - Fetch and open the puzzle input

 - Submit an answer to AoC

Inspired by https://github.com/keegancsmith/advent/