Support for non-armored detached signatures

[smira]

Fixes #773

Description of the Change

Seems like some repos start providing non-ASCII armored detached signatures, and aptly has to support them.

Checklist

• unit-test added (if change is algorithm)
• functional test added/updated (if change is functional)
• man page updated (if applicable)
• bash completion updated (if applicable)
• documentation updated
• author name in AUTHORS

[codecov]

Codecov Report

Merging #780 into master will increase coverage by 0.08%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master     #780      +/-   ##
==========================================
+ Coverage   64.05%   64.13%   +0.08%     
==========================================
  Files          50       50              
  Lines        6326     6329       +3     
==========================================
+ Hits         4052     4059       +7     
+ Misses       1778     1776       -2     
+ Partials      496      494       -2

Impacted Files	Coverage Δ
pgp/internal.go	47.61% <100%> (+0.63%)	⬆️
pgp/openpgp.go	51.47% <0%> (+2.94%)	⬆️

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[sliverc]

LGTM