Core help command

It'd really be nice if we had a help command for the bot. I'm thinking it would find and display the help for any plugin available, using any available plugin's name. Something like:

```bash
command: [botname] help
response: Type [botname] help [topic] for detailed help on a topic. Available topics are 'Factoid', 'Google', 'Seen'.
```

Then we could have each plugin have an `exports.help = "Help text"` and use that for the next response:

```bash
command: [botname] help Google
response: Type [botname] google [search-term], [botname] wikipedia [search-term] to get the first result found.
```

I don't know if we'd require `exports.help` for each plugin or we would just keep it a convention, but either way it would certainly make this bot more usable.

Thoughts?