Simple rewrite

[dnlgrv]

Revisiting this gem's code after a year was difficult to look at, so I decided to do a quick rewrite of everything that matters and get it all up to speed again.

Nothing significant has changed, except the gem is a little more "dumb" now instead of trying to catch edge cases or if you've skipped setup steps.

• Configuration is gone
• Solution methods have been renamed from #part1 and #part2 to #part_1 and #part_2 respectively
• Input is loaded into your solutions using #text_input, previously #load_input
• advent generate has been renamed to advent new