Improve the Config implementation a bit

* Don't export functions from Config, instead, return them in a table.
* More idiomatic method for requiring a json library.
* Move Löve version extraction to a function here.
* Add a function to set the window title.
* Print better errors when trying to load the json library and
  configuration file.
* Save Löve version information globally.
* Add support for 0.8 during the config call.
* Added support for setting the release flag as an option. When not in
  release mode, "(development)" is added to the window title.
* LP's __love version check now uses the global.
* When LP is loaded, it will check if the config was called, and setup
  LunoPunk defaults if it hasn't.