Fixes and docs for the ranger plugin

[ilyagr]

Makes Ranger plugin use the standard $ZLUA_LUAEXE and $ZLUA_SCRIPT
environment variables that z.lua, fixes error handling, adds docs.

[ilyagr]

• Makes Ranger plugin use the standard $ZLUA_LUAEXE and $ZLUA_SCRIPT
  environment variables that z.lua itself uses so that it works out of the box
  (no need to explicitly set RANGER_ZLUA).

• Better error handling; errors loading plugin no longer cause
  ranger to quit entirely.

• Some minor stylistic changes to the Python code to make it more readable.

• Briefly document the plugin

[ilyagr]

I split this PR into two commits: the first one has all the important changes in a small diff. The second one consists only of stylistic changes but has a larger diff.

Let me know if I should combine them into one commit, remove the second one, etc.

[skywind3000]

thanks

[ilyagr]

Thank you! I also realized there's a FAQ entry about ranger integration. You probably should delete the portion that says

and set environment variable in your shell rc: export RANGER_ZLUA="/path/to/z.lua"

Or, perhaps, just replace the entry with a link to the relevant portion of README.md.