allow plugin to unload itself, add `Meta::ReloadPlugin()`

[ezio416]

A plugin can be unloaded from another plugin, but why not itself? There are two use cases I could see this being useful in:

• reload itself to aid in development (kind of a duplicate of #154 except I think it makes more sense for it to be Meta::ReloadPlugin(plugin) instead of a member of Meta::Plugin)
• if a user has inadequate permissions then we can just stop the plugin completely to:
  • save some resources
  • save a bit of development work, i.e. if a plugin requires club access then I have to write in a bunch of things to hide the plugin from the user, whereas if it were unloaded then I don't need to do anything

For Meta::ReloadPlugin(), if implemented I feel it should return a new Meta::Plugin

[codecat]

I think the permissions thing could potentially be done in info.toml as well. 🤔