Add support for prerm and postinst files

[todo]

https://github.com/trickypr/libdeb/blob/cca5dc29be9845729547faa5509974a285263e1c/src/deb.rs#L51-L55

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This issue was generated by todo based on a todo comment in cca5dc2. It's been assigned to @trickypr because they committed the code.