Add Pipe Middleware

[CHH]

Add a Pipe Middleware which captures requests to a certain prefix (e.g. by default /_pipe) and responds with the asset just like Pipe\Server. This Middleware implements the HttpKernelInterface and should be usable within Middleware pipelines.

Benefits:

• Barebones integration with Symfony possible by decorating Symfony's HttpKernel, like the HttpCache does.
• Common class between the Silex and Symfony integrations (eventually)