new way to pass the configuration to the encodings

[bjohansebas]

Problem

In the current design of how the properties are passed, the options for deflate and gzip are the same. This means that if you want to have different configurations for these two encodings, you won’t be able to. Additionally, you can’t disable these encodings, which in cases like #25, might require disabling a specific encoding.

Solution

The solution is to handle the configurations separately, where each encoding has its own configuration, and also to provide the ability to disable an encoding by passing false.

// OLD
compression({ level: -1, chunkSize: 7000 }) // It applies to both deflate and gzip. 
// Brotli has its own separate configuration.

// NEW
compression({ encodings: { gzip: { level: -1, chunkSize: 7000}  }) // It only applies to gzip.
compression({ encodings: { gzip: false   }) // Disable gzip.

Suggestions about this new API are welcome.

Closes #220
Closes #25
Closes #27