Add support for custom emojis

[ozxybox]

It would be super cool if we could embed stuff like game icons as emojis. It seems like it would be doable, we would just need to intercept emojis before they hit twemoji.

Emojis should be stored in the Wiki repo under assets/emojis/ and get picked up automatically.
Adding a file like assets/emojis/game_hl2.svg should make the emoji :game_hl2: work from the docs.