passing regex flags to learn

[caub]

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[Zirak]

So /learn is broken from the get go for anything that's non-trivial. Pretty much all learned commands can be broken down into two categories:

• Commands which reply with a constant
• Commands which require logic

I've sinned in trying to create a macro language for the 2nd via some regexps. That was a mistake. With the introduction of eval there should've been a version of /learn which just lets you run js, maybe with some helper functions.
This commit, while good in faith, propagates the mistake further. Now /learns syntax will be even more obscure, with the input regexp's flags coming after the description.

I appreciate the PR, but reading this alongside #252 made me realize that /learn should be burned to the ground (I'll say why 252 happens separately) and be replaced with something like /learnEval.

:(