Add spoiler and strikethrough metacharacters to escaped chars

[ephphatha]

forgot this one originally, people could trigger spoiler tags by starting and ending their name with two pipes

[StephenCWills]

Should we escape tilde for strikethrough?

[ephphatha]

Completely missed that one, there's probably more but I couldn't find documentation of what discord markdown actually supports when we were initially discussing it. Guess we'll find out if it comes up :D