Use pinentry for passphrase input if available

[str4d]

Closes #55.

[codecov]

Codecov Report

Merging #64 into master will decrease coverage by 0.59%.
The diff coverage is 0%.

@@            Coverage Diff            @@
##           master      #64     +/-   ##
=========================================
- Coverage      49%   48.41%   -0.6%     
=========================================
  Files          22       22             
  Lines        1716     1737     +21     
=========================================
  Hits          841      841             
- Misses        875      896     +21

Impacted Files	Coverage Δ
rage/src/bin/rage-mount/main.rs	0% <ø> (ø)	⬆️
rage/src/bin/rage/main.rs	0% <0%> (ø)	⬆️
rage/src/bin/rage/error.rs	0% <0%> (ø)	⬆️
age/src/cli_common.rs	0% <0%> (ø)	⬆️

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