Add loading script to simplify loading different models

[orlade-anz]

If a command line arg is provided, it is treated as a path to a Sysl model load instead of the default (model.pb).

[codecov]

Codecov Report

Merging #970 into master will increase coverage by 1.18%.
The diff coverage is 88.12%.

@@            Coverage Diff             @@
##           master     #970      +/-   ##
==========================================
+ Coverage   83.35%   84.53%   +1.18%     
==========================================
  Files          73       75       +2     
  Lines       10609    10503     -106     
==========================================
+ Hits         8843     8879      +36     
+ Misses       1432     1313     -119     
+ Partials      334      311      -23     

Impacted Files	Coverage Δ
pkg/pbutil/output.go	76.00% <ø> (-0.48%)	⬇️
pkg/syslwrapper/app.go	93.44% <ø> (-0.05%)	⬇️
pkg/syslwrapper/test_helper.go	57.89% <ø> (-1.42%)	⬇️
pkg/importer/importer.go	63.15% <63.15%> (ø)
pkg/mod/gomod.go	73.68% <66.66%> (+1.27%)	⬆️
pkg/mod/module.go	66.66% <72.72%> (+0.95%)	⬆️
pkg/parse/parse.go	85.34% <73.07%> (+1.37%)	⬆️
pkg/importer/openapi.go	85.96% <85.18%> (-3.48%)	⬇️
pkg/parse/listener_impl.go	89.62% <86.48%> (+0.11%)	⬆️
pkg/importer/swagger.go	81.48% <88.88%> (+1.88%)	⬆️
... and 22 more