Implement case-sensitive command option

[ghost]

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[eritbh]

This is a good start, thanks for the contribution! It's missing a couple things that I'll address in further commits:

• I'd prefer the name of the property to be caseSensitiveCommands (plural).
• Command names should be converted to lower case when they're added to the bot, to avoid accidentally creating unusable commands through including capital letters in command names. This actually adds way more complexity than I realized, so it's good as it is. I'll add some documentation saying that command names in code must be all lowercase when this flag is false.