Handle signing-only subkeys

[crazy-max]

Fixes #58

[codecov]

Codecov Report

Merging #112 (b7d4b7c) into master (1fb7d85) will decrease coverage by 0.17%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master     #112      +/-   ##
==========================================
- Coverage   81.48%   81.30%   -0.18%     
==========================================
  Files           3        3              
  Lines         108      107       -1     
  Branches       24       23       -1     
==========================================
- Hits           88       87       -1     
  Misses         11       11              
  Partials        9        9              

Impacted Files	Coverage Δ
src/context.ts	83.33% <ø> (ø)
src/openpgp.ts	100.00% <ø> (ø)

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