Fix placeholders being lost upon using /papi reload

[akshualy]

Someone else brought to my attention that placeholders would return %tm_tokens% instead of the balance randomly.

The reason for that is that if you use an internal class to register placeholders, you have to tell PlaceholderAPI that your placeholders should not be unloaded upon calling /papi reload.

Before:

After:

[Realizedd]

Included in 340464fb19752e928f3340817c03aaeeeb42db3b. Thanks for the contribution :)