add stancflags from make/local

[rok-cesnovar]

Summary

Fixes #683

Copyright and Licensing

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Rok Češnovar

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• Code: BSD 3-clause (https://opensource.org/licenses/BSD-3-Clause)
• Documentation: CC-BY 4.0 (https://creativecommons.org/licenses/by/4.0/)

[andrjohns]

@rok-cesnovar Thought I'd update this PR as well (hope you don't mind!), since the facility for reading flags from make/local was introduced with the model methods and expose functions PRs

[rok-cesnovar]

I dont mind at all. Thank you!

[rok-cesnovar]

Will merge once tests pass. You are a rockstar :)