[FEATURE] Server: Add handling for custom middleware

[RandomByte]

As per RFC 0005: SAP/ui5-tooling#151

[RandomByte]

We should consider releasing this as a 2.0 since the index.js and the signatures of the middleware modules changed significantly.

[matz3]

We could also deprecate the old middleware (exported via index.js) and map the argument names, right?
This way we wouldn't need to make a 2.0 and give consumers a chance to adopt their code in advance (although we never documented the exported middleware as public API).

[RandomByte]

We could also deprecate the old middleware (exported via index.js) and map the argument names, right?

Done in latest change

[coveralls]

Coverage increased (+0.7%) to 93.529% when pulling 8859ab0 on server-middleware-extensibility into e726d68 on master.