Usage
1. Download `aoc_to_markdown-1.0.6-an+fx.xpi`.
2. Install using Firefox's `Install Add-on From File...` option.
3. Navigate to any challenge page (e.g. https://adventofcode.com/2017/day/14).
4. The `Advent of Code to Markdown` button will show up in the address bar.
5. Click the button and it will pop up a `Save As...` dialog to save the output.