feat: more config options

[matsp]

Some ideas that could be configureable with passed options:

• define which asset types should be "pushed" e.g. css only, js only or both (in most cases it's better to just push css - see [2]
• rel attribute should be optional e.g. preload (server maybe skip pushing in favor of browser cache - see [1]
• ...

Resources:

• [1] https://w3c.github.io/preload/#link-element-interface-extensions
• [2] https://www.smashingmagazine.com/2017/04/guide-http2-server-push/