Refactored image search commands, safe search defaults to off, added NSFW back in

Pulled all the image search code into one block to reduce a bunch of duplication and ambiguous conditions.

Also now defaults to using safe search since most of us are at work while using the bot.

Valid image search commands:

`image me TEXT`
`image first TEXT`
`image nth NUM TEXT`
`image random`

`nsfw` can be added before the image commands to turn off safe search.

Changed it so that the line must start with the command so the bot won't jump in when talking about this code.