Two argument ustrip for logarithmic units

[gustaphe]

Allows ustrip(u"dB", 3u"B") and ustrip(u"dBV", 1u"dBµV").

No changes to documentation, this feels like expected behavior from the current description of ustrip.

[codecov-commenter]

Codecov Report

Merging #535 (0daf2a6) into master (960e09e) will increase coverage by 0.07%. The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master     #535      +/-   ##
==========================================
+ Coverage   88.19%   88.26%   +0.07%     
==========================================
  Files          16       16              
  Lines        1448     1449       +1     
==========================================
+ Hits         1277     1279       +2     
+ Misses        171      170       -1     

Impacted Files	Coverage Δ
src/logarithm.jl	78.77% <100.00%> (+0.49%)	:arrow_up:

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