Add option --expand with skosify

[codecov-io]

Codecov Report

Merging #45 into master will increase coverage by 0.99%. The diff coverage is 77.14%.

@@            Coverage Diff             @@
##           master      #45      +/-   ##
==========================================
+ Coverage   78.73%   79.72%   +0.99%     
==========================================
  Files           5        6       +1     
  Lines         710      730      +20     
==========================================
+ Hits          559      582      +23     
+ Misses        151      148       -3

Impacted Files	Coverage Δ
mc2skos/mc2skos.py	56.39% <58.82%> (+2.33%)	:arrow_up:
mc2skos/reader.py	94.44% <94.44%> (ø)

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[danmichaelo]

Sorry it took some time.

[nichtich]

Slightly changed files at #47