Improve message placeholder support

[GeorgH93]

Currently placeholders in messages are handled through String.format this approach is rather limited. Problems with the current implementation:

• It's impossible to support RGB colors in placeholders
• Formatting in placeholders is broken if it is split across 2 lines
• Adding JSON in placeholders is impossible
• String.format does not allow for format caching
• It's possible to create broken messages way to easy
• PAPI integration is really unsafe
• Data for placeholders has to be retrieved, even when it's not used

The new interface should use proper placeholders instead of replaceAll.
There will be a IPlaceholderFormatter interface, that gets implemented by the user and it retrieves and formats the placeholder on demand. The placeholder resolver will be registered with the message via this method: void registerPlaceholder(String placeholderName, IPlaceholderResolver placeholderResolver)