Signature notation

[JohannesEbke]

This PR adds a signature notation to the created signatures.

[codecov-io]

Codecov Report

Merging #43 into master will increase coverage by 0.13%.
The diff coverage is 59.6%.

@@            Coverage Diff             @@
##           master      #43      +/-   ##
==========================================
+ Coverage   59.37%   59.51%   +0.13%     
==========================================
  Files          14       41      +27     
  Lines         832     4537    +3705     
==========================================
+ Hits          494     2700    +2206     
- Misses        252     1390    +1138     
- Partials       86      447     +361

Impacted Files	Coverage Δ
....org/x/crypto/openpgp/packet/one_pass_signature.go	0% <0%> (ø)
...ndor/golang.org/x/crypto/openpgp/packet/literal.go	0% <0%> (ø)
...endor/golang.org/x/crypto/openpgp/packet/reader.go	0% <0%> (ø)
mail/multipart.go	66.66% <100%> (-0.38%)	⬇️
...r/golang.org/x/crypto/openpgp/packet/compressed.go	15.55% <15.55%> (ø)
...olang.org/x/crypto/openpgp/packet/public_key_v3.go	36.42% <36.42%> (ø)
...olang.org/x/crypto/openpgp/packet/userattribute.go	37.5% <37.5%> (ø)
...golang.org/x/crypto/openpgp/packet/signature_v3.go	42.25% <42.25%> (ø)
...endor/golang.org/x/crypto/openpgp/packet/config.go	45.83% <45.83%> (ø)
...x/crypto/openpgp/packet/symmetrically_encrypted.go	47.48% <47.48%> (ø)
... and 46 more

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