Several PluginManager improvements

* load/unload/reload are no longer restricted to plugins that exist
  on startup
* Names passed to DFHACK_PLUGIN must match the plugin's filename
  (remotefortressreader vs RemoteFortressReader, counters vs probe)
* "plug" output lists all plugins and state/command information
* Deleted plugins can be reloaded again if they are replaced
* load/unload/reload don't fail silently with broken plugins
* Built-in commands are recognized internally (e.g. "help help"
  does not display "help is not a recognized command"), although help
  for them is not yet implemented
* New command: "type" (bash-like) - shows where/how a command is
  implemented
* "plug" can accept multiple plugin names
* "ls" displays more information about unloaded/unrecognized plugins
* "load all" changed to "load -all" (or "load --all", "load -a", ...)