add --ignore FILE option

Allows you to specify a file listing packages to ignore. 

Very basic: does not know anything about dependencies, so you have to explicitly list all the packages you don't want, including their dependencies. It's kind of a pain, but it works, and I would assume this would be a very common need (as indicated by @pwalsh 's fork too).

Also changed it to parse argv with OptionParser (easier when you have >1 option)