Support the PUG XML interface for substructure queries

[timholy]

This was more painful, but very handy for long-running, complex searches.

[codecov]

Codecov Report

Merging #7 (f9514f9) into master (45cedb5) will decrease coverage by 1.08%.
The diff coverage is 86.73%.

@@            Coverage Diff             @@
##           master       #7      +/-   ##
==========================================
- Coverage   90.19%   89.11%   -1.09%     
==========================================
  Files           3        4       +1     
  Lines          51      147      +96     
==========================================
+ Hits           46      131      +85     
- Misses          5       16      +11     

Impacted Files	Coverage Δ
src/PubChemCrawler.jl	100.00% <ø> (ø)
src/query.jl	90.90% <66.66%> (+1.25%)	⬆️
src/pugxml.jl	88.04% <88.04%> (ø)

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