Add Grok format support

[iximiuz]

ELK's Grok is a powerful and quite concise format to parse arbitrary text and structure it. pq partially reinvents it by combining a regex decoder with a map filter. But having a native Grok support would help the adoption - there are likely people already familiar with Grok.

[iximiuz]

Dissect also can be added as a simpler and faster alternative to Grok.