--pluginpath refactor

with this patch --pluginpath [contrib dir path] will correctly find the lib/logstash dir and also find the contrib gems and add it to GEM_PATH. It follows the same previous logic of prepending these paths so that a plugin from --pluginpath would overwrite one in the logstash distribution.

My concern with this is having multiple GEM_PATHs could cause gem conflicts. I DID NOT test this yet, I am pushing this PR mainly to get feedback on this idea and maybe get some testing.