clamd: load config once with callback

* moved config loads into into register w/callbacks …
* enable 'use strict'
* moved skip_list & skip_list_exclude from globals into plugin object
* load_excludes & load_clamd_ini are loaded in register with callbacks
* add more tests
FYI: this is unnecessary in plugins as it's implicit.   See https://github.com/baudehlo/Haraka/blob/master/plugins.js#L134