Misc: give better help when command isn't found but coincides w/ the name of a loaded plugin

As suggested by #912. Provided a plugin *is not marked as private*, trying to do `help pluginname` will give:

```
<synnerjee> help mATH
<Atlas> Error: There is no command "math". However, "Math" is the name of a loaded plugin, and you may be able to find its provided commands using 'list Math'.
```
It is case insensitive also.