Expose intersection joint type for shell

[adam-urbanczyk]

This PR will enable keeping sharp edges when shelling outwards

res1= cq.Workplane().box(1,1,1).faces('>Z').shell(.4,kind='intersection')

res2= cq.Workplane(origin=(3,0,0)).box(1,1,1).faces('>Z').faces('>Z').shell(.4,kind='arc')

[codecov]

Codecov Report

Merging #413 into master will increase coverage by 0.00%.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##           master     #413   +/-   ##
=======================================
  Coverage   93.56%   93.56%           
=======================================
  Files          19       19           
  Lines        5142     5147    +5     
  Branches      530      530           
=======================================
+ Hits         4811     4816    +5     
  Misses        209      209           
  Partials      122      122           

Impacted Files	Coverage Δ
cadquery/cq.py	87.93% <100.00%> (ø)
cadquery/occ_impl/shapes.py	90.68% <100.00%> (+0.01%)	⬆️
tests/test_cadquery.py	98.98% <100.00%> (+<0.01%)	⬆️

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[jmwright]

I think it looks good, thanks for adding this.