Exclude support, fixes #5

This PR adds support for excluding files (and directories) as described in #5 and originally implemented in #15.

At the moment I'm unsure if `Package::isBlacklisted()` is implemented correctly to exclude files from being added in the `ExplicitBundler::addFile()` method. It should be possible to exclude directories which then must be respected for the files defined via autoload's `files` and `classmap` definitions. From the code it seems like it's different from the filter defined in `Package::getBlacklistFilter()`, nevertheless I suppose the same functionality is already there, but hidden in how Symfony's `Finder` works.