Added support for trailing commas within libconfig arrays

- A trailing comma in conf/plugins.conf will no longer cause a parse
  error. This makes it easier to comment out plugins to disable them.
- conf/plugins.conf has been updated to reflect this change.

Signed-off-by: Haru <haru@dotalux.com>
Added 'buildclean' target to all makefiles

- By invoking 'make buildclean', you can clean up all the build
  intermediate (object) files, without deleting any of the built
  executables and plugins.
- To delete everything, use 'make clean', as you've always done.

Signed-off-by: Haru <haru@dotalux.com>
Merge branch 'master' of https://github.com/HerculesWS/Hercules