Support longer descriptions, using first sentence as short description

It's useful to have longer help descriptions for commands. It's useful to have short descriptions as a summary of commands.

Tools such as Javadoc and click allow long descriptions and use the first sentence as the short description.

On the help page for the full bot, use the first sentence of the description as a summary
On the help page for a specific command, use the full description (this also means it's probably better off not in the title of the embed, but just as plain text)

Also possible is supporting `summary` in the @CommandHandler, so that it can be explicitly given if preferred, but defaulting to the first sentence of `description` if it's not given.