Use alternatives instead of Plack::MIME

I am a little unhappy with Plack::MIME content-types, especially when using Plack::Middleware::Static. Sometimes I want a improved MIME-type detection, like File::MimeInfo::Magic.

I added a content_type attribute to Plack::Middleware::Static, which can be a sub like 
```content_type => sub { mimetype($_[0]) },```

The content_type is now handed to Plack::App::File, which is extended to evaluate the CODE reference if content_type is one. For the primary use of Plack::App::File it might be not necessary to handle code references for content_type, but I think it is the best place to deal with it and to provide this feature to other components reusing Plack::App::File.