Added some features & fixed PlaceholderAPI

[codingcat2468]

Hi, I really like this project and wanted to contribute a bit by adding the following things:

• I fixed PlaceholderAPI not working:
  Basically, the reason that heads don't show up correctly with PlaceholderAPI is that the default Minecraft chat format doesn't support changing the font of some parts of a message, which is only possible using chat components, which PlaceholderAPI doesn't support. I solved this issue by just not using a custom font for this at all, but rather adding the custom unicode pixels to the default Minecraft font, which makes them useable everywhere, without using the custom font in just the head.
• I made the placeholder able to display other people's head using thier username: You can now either use %chathead_me% to get your own head or %chathead_<username>% to get the head of the player with the given username.
• I also made the join & leave chat messages work without having to uncomment registerExamples() in the onEnable method and added support for showing player heads in normal chat and death messages.
• The resource pack can now automatically be downloaded from github and will be applied to the players without them needing to download anything.
• The plugin now has a simple configuration to enable / disable player heads in certain message types and the automatic resource pack download. (I also updated the README file to explain some of these changes)

I know that's quite a lot of changes, I hope it's not too much and that my changes are helpful!