Add support for compressed zlp files

[NickCondron]

Slippc implements a custom compression scheme for slp files: https://github.com/pcrain/slippc#compression

The format filetype is called zlp. Supporting reading/writing to this format would be really useful.

[hohav]

I'm not interested in maintaining parallel code for the zlp format, especially given that peppi's own format is intended to provide the same benefits plus more. And users can of course call slippc themselves to convert a replay to SLP before parsing.