Allow more than one plugin folder

This Pull Request allows the user to decide how many plugin folders he wasnt and where he would like them located. This aids in organisation of plugins - You could maybe have a structure like this
- essentials/
- worldmanagement
- worldmanagement/Grief
- chat
- chat/staff
- ../miscOneTimeConfigThenDoesntWriteAnythingPlugins

If the folder specified in config doesn't exist then it will create it and warn the user
If the Config contains an empty list of plugin folders, it will use the default  and warn the user
If the Config option doesn't exist it will create it with just a 'plugins' folder.

The config is under settings and is just a list of folders:
>   plugin-folders:
>  \- essentials
>  \- worldmanagement
>  \- worldmanagement/Grief
>  \- ../MyOtherFolder