Restructure API so InvseeAPI instances are created per api user

[Jannyboy11]

We would need to add the following method to com.janboerman.invsee.spigot.common.api.InvseePlusPlus:

public InvseeAPI getApi(org.bukkit.plugin.Plugin plugin);

This allows plugins to aquire the InvseeAPI instance of other plugins, and modify its default settings. The existing method InvseeAPI getApi() will be retrofitted to return the InvseeAPI instance of InvSeePlusPlus itself. This would allow api consumers to modify the default behaviour of the built-in /invsee and /endersee commands. (Note: these command must make use of the InvseeAPI instance for the default CreationOptions, which they currently don't use.)