コマンドラインから指定したプラグインディレクトリがPathnameオブジェクト化されていませんが、Plugin.load_pluginメソッドではPathnameオブジェクトであることが前提となっている為例外が発生します。
% ruby -Ku -I./pragger-svn ./pragger-svn/pragger.rb -p ./my-pragger-files/plugin -l
/usr/local/lib/ruby/1.8/pathname.rb:695:in `relative_path_from': undefined method `cleanpath' for "./my-pragger-files/plugin":String (NoMethodError)
from ./pragger-svn/pragger.rb:11:in `load_plugins'
from ./pragger-svn/pragger.rb:9:in `each'
from ./pragger-svn/pragger.rb:9:in `load_plugins'
from ./pragger-svn/pragger.rb:29
from /usr/local/lib/ruby/1.8/optparse.rb:1247:in `call'
from /usr/local/lib/ruby/1.8/optparse.rb:1247:in `order!'
from /usr/local/lib/ruby/1.8/optparse.rb:1205:in `catch'
from /usr/local/lib/ruby/1.8/optparse.rb:1205:in `order!'
from /usr/local/lib/ruby/1.8/optparse.rb:1279:in `permute!'
from /usr/local/lib/ruby/1.8/optparse.rb:1300:in `parse!'
from ./pragger-svn/pragger.rb:32
from /usr/local/lib/ruby/1.8/optparse.rb:755:in `initialize'
from ./pragger-svn/pragger.rb:27:in `new'
from ./pragger-svn/pragger.rb:27
