Add 'stringconfig set' command

[jimcullenaus]

Add a feature that allows @PockyBot stringconfig set <key> <value> to set the stringconfig for <key> to <value> in unambiguous cases.

• If <key> is not yet set, set it to <value>.
• If <key> is set with one associated value, update that value to <value>.
• If <key> is set with two or more values, return an error and state what those values are.

[Evangelinexx]

Would 'update' be a better term to use? I'm not sure how intuitive 'set' is as a keyword

[jimcullenaus]

My thinking was that it's sort of equivalent to an HTTP PUT request. Update sounds more like a PATCH, it'll update what already exists but won't create it if it doesn't.