Add option for TLS

[iconnor]

Goal: follow mosquitto_sub --help as closely as possible.

Naturally, mosquitto_sub have different options and will negotiate SSL for you. Paho expects you to call tls_set. So, I borrowed the insecure flag from mosquitto_sub to call if this is on or off.

As you know, I am using let's encrypt, so I don't need to pass the ca_certs to get it all working. I left the option if you want but have not tested it. If you prefer, I can remove those and leave the insecure option as the only one.

[pep8speaks]

Hello @iconnor! Thanks for updating this PR. We checked the lines you've touched for PEP 8 issues, and found:

There are currently no PEP 8 issues detected in this Pull Request. Cheers! :beers:

Comment last updated at 2021-04-04 12:11:07 UTC

[codecov[bot]]

Codecov Report

Merging #32 (ead497c) into master (4294158) will increase coverage by 0.06%. The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master      #32      +/-   ##
==========================================
+ Coverage   95.77%   95.83%   +0.06%     
==========================================
  Files           6        6              
  Lines         828      841      +13     
==========================================
+ Hits          793      806      +13     
  Misses         35       35              

Impacted Files	Coverage Δ
modbus4mqtt/modbus4mqtt.py	94.50% <100.00%> (+0.38%)	:arrow_up:
tests/test_mqtt.py	97.40% <0.00%> (-1.16%)	:arrow_down:
tests/test_modbus.py	97.14% <0.00%> (+2.28%)	:arrow_up:

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[iconnor]

You set a pretty high bar - using the pep8 tool, it does mean new PRs have to clean up any code in files they touch. I am not against it, just letting you know why the PR kind of exploded in scope.