Exclude support, fixes #5

[mikey179]

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.

[clue]

Awesome changeset! I'd love to get this in! :clap:

At the moment I'm unsure if Package::isBlacklisted() is implemented correctly

I suppose this is still relevant? Is there anything I can do to help push this forward?

IIRC either definition should be sufficient and there's little point in providing two definitions for this. I suppose can be verified through a few test cases?