Update ext_plugin.py

If a .conf file exists in the config_dir and it's first section has an enable_plugin key and enable_plugin is not true, the plugin name should be appended to _disabled_plugins, not the last section name from the early section scan (e.g. "log").

Copy-paste error, methinks.
Merge pull request #244 from delitescere/patch-1

Fix bug in ext_plugin.py