Configuration options for Rack::Deflate

Rack::Deflater currently does not support any configuration options which makes it cumbersome to control.

This pull request adds support for the following options:
* __include__ - an array of content types enabled for compression
* __if__ - a lambda for choosing whether to deflate based on current execution scope (request, status, body, or headers)

Examples:
```use Rack::Deflater, include: %w(text/json application/json)```
```use Rack::Deflater, if: lambda { |env, status, headers, body| body.length > 512 } ```

**EDIT**: description was update based on a PR discussion