OnServerInitialized bug fix

Noticed that the hook OnServerInitialized isn't running when a plugin is
loaded through the console command oxide.load <plugin>, these changes
allow the rust extension to access the core's pluginmanager to get the
plugin after loading through oxide.load and runs the OnServerInitialized
hook for that plugin.
Merge pull request #24 from Mughisi/OnServerInitialized-changes

OnServerInitialized bug fix