Default umask to restrict permissions on file

[ewjoachim]

Closes #195

Successful PR Checklist:

• Tests (units, integration, 100% coverage)
  • (not applicable?)
• Documentation
  • (not applicable?)
• Had a good time contributing?

[codecov]

Codecov Report

Merging #196 (8c781fa) into master (75dcabd) will not change coverage.
The diff coverage is 100.00%.

@@            Coverage Diff            @@
##            master      #196   +/-   ##
=========================================
  Coverage   100.00%   100.00%           
=========================================
  Files           13        13           
  Lines          981       978    -3     
  Branches       130       128    -2     
=========================================
- Hits           981       978    -3     

Impacted Files	Coverage Δ
vault_cli/cli.py	100.00% <100.00%> (ø)

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