Add very simple shell completion using argcomplete

[ianmcorvidae]

Fixes #640 in a rudimentary sort of fashion.

This could probably be improved upon by way of the process shown in the argcomplete docs -- but, it may also be the kind of thing that would depend on local persistence to be very efficient. This does at least add basic completion, which can be enabled with: eval "$(register-python-argcomplete meshtastic)" (which can be put into a bashrc or similar). I'm unclear if argcomplete's support for global completion will work at present.

[codecov]

Codecov Report

Attention: Patch coverage is 77.77778% with 2 lines in your changes missing coverage. Please review.

Project coverage is 61.01%. Comparing base (749c6a7) to head (0b15453).
Report is 3 commits behind head on master.

Files with missing lines	Patch %	Lines
meshtastic/__main__.py	77.77%	2 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #721      +/-   ##
==========================================
+ Coverage   60.99%   61.01%   +0.02%     
==========================================
  Files          24       24              
  Lines        3899     3907       +8     
==========================================
+ Hits         2378     2384       +6     
- Misses       1521     1523       +2     

Flag	Coverage Δ
unittests	61.01% <77.77%> (+0.02%)	⬆️

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