Display compressed files during text preview

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.

Start at the :toggle_preview implementation and trace how selected files are sent to the text preview. Compare the behavior with the issue's zmore example, including .gz, .z, and .Z files. Done means selecting helloworld.txt.gz and toggling the preview displays its unzipped text instead of binary data.

Written by the indexing model from the issue text.