Merge blacklist and whitelist codebases

[chy-causer]

Hello,

Firstly thanks for the Zabbix template! Very useful.

I've created a patch which does a few things

1) It merges Whitelist and Blacklist functionality into one codepath, resulting in 274 fewer lines 2) Slightly cleans up the filter by using the -f flag in grep 3) Clarifies in the README that you can use both whitelists and blacklists and what the behaviour should be when you do. Also that the filters are regular expressions 4) Allows you to run the script with no filters at all!

Any questions please let me know.

[MogiePete]

@chy-causer

I appreciate the contribution and the code cleanup. I have been running your PR for a couple days and all looks well.

Thanks again!