Display compressed files during text preview

[jjvliu]

Many Linux systems ship with zmore as a filter that transparently passes gzipped files (.gz, .z, .Z) through gzip --cdfq -- "$FILE" before sending them to the pager (e.g. less on my system). It would be helpful to have :toggle_preview mimic this behavior by default instead of displaying binary.

Example: select helloworld.txt.gz and :toggle_preview. Desired output is the unzipped text, not the binary.