WIP: only evaluate fragment includes

[erdos]

Add a system option to only evaluate fragment include directives.

[codecov-io]

Codecov Report

Merging #31 into master will increase coverage by 0.33%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master      #31      +/-   ##
==========================================
+ Coverage   91.82%   92.15%   +0.33%     
==========================================
  Files          19       19              
  Lines        1406     1427      +21     
  Branches       84       82       -2     
==========================================
+ Hits         1291     1315      +24     
+ Misses         31       30       -1     
+ Partials       84       82       -2

Impacted Files	Coverage Δ
src/stencil/tokenizer.clj	96.87% <100%> (+2.5%)	⬆️
src/stencil/model.clj	93.63% <100%> (+0.04%)	⬆️
src/stencil/merger.clj	100% <100%> (ø)	⬆️
src/stencil/util.clj	96.72% <100%> (+0.05%)	⬆️
src/stencil/process.clj	78.04% <100%> (+1.12%)	⬆️
src/stencil/cleanup.clj	93.66% <0%> (+0.7%)	⬆️

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