load plugins by config

The plugins are now loaded if their are in the plugins config. For
example to load the op plugin it is required for the config to contain:

plugins:
  op:
use the name of the config section as plugin objects
refactor list_plugins, use String.constantize
unpack values while iterating over plugins config
replace list_plugins with a mapping of keys in plugin config
Merge pull request #3 from relekang/plugin-loading

load plugins by config