Support for shell completion

[samschott]

Closes #96.

[codecov]

Codecov Report

Merging #386 (1a2377d) into master (d8cb706) will decrease coverage by 0.04%.
The diff coverage is 20.27%.

@@            Coverage Diff             @@
##           master     #386      +/-   ##
==========================================
- Coverage   78.18%   78.14%   -0.05%     
==========================================
  Files          32       32              
  Lines        6602     6666      +64     
==========================================
+ Hits         5162     5209      +47     
- Misses       1440     1457      +17     

Flag	Coverage Δ
pytest	78.14% <20.27%> (-0.05%)	⬇️

Flags with carried forward coverage won't be shown. Click here to find out more.

Impacted Files	Coverage Δ
src/maestral/cli.py	51.31% <15.71%> (-2.46%)	⬇️
src/maestral/config/main.py	94.44% <100.00%> (+0.69%)	⬆️
src/maestral/daemon.py	81.81% <0.00%> (ø)
src/maestral/client.py	85.56% <0.00%> (+0.13%)	⬆️
src/maestral/utils/orm.py	91.76% <0.00%> (+0.41%)	⬆️
src/maestral/main.py	76.67% <0.00%> (+0.53%)	⬆️
src/maestral/sync.py	87.56% <0.00%> (+1.44%)	⬆️
src/maestral/manager.py	85.82% <0.00%> (+2.23%)	⬆️

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