Add support for RFC 7508

[etingof]

See #99

[codecov-io]

Codecov Report

Merging #102 into master will increase coverage by <.01%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master     #102      +/-   ##
==========================================
+ Coverage   99.35%   99.36%   +<.01%     
==========================================
  Files          90       91       +1     
  Lines        5774     5801      +27     
==========================================
+ Hits         5737     5764      +27     
  Misses         37       37

Impacted Files	Coverage Δ
pyasn1_modules/rfc7508.py	100% <100%> (ø)

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