Introduced a filesystem tag and use it within the PluginCompiler

I decided to go a little bit a different way than you proposed. Instead of wildcards, you can now use the property `filesystem` on the plugin tag, which makes it a lot easier to use. I also introduced a filesystem tag, which will be used afterwards anyways.

The reason I made a pull request to your branch is that I really like your idea and I want you being properly mentioned as a contributor in the main bundle. So if you merge my pull request to your fork, I'll afterwards merge it to the OneupFlysystemBundle. :)

Have nice weekend!