simple-markdown: prep towards perseus-independence
simple-markdown: prep towards perseus-independence

Summary:
Adds an explicit license, and handles dependencies/exports
even if `require` doesn't exist

Test plan:
`make test` still works
cross fingers for what happens when `require` doesn't exist :)

Auditors: emily

daukantas/perseus,chirilo/perseus,sachgits/perseus,alexristich/perseus,sachgits/perseus,iamchenxin/perseus,alexristich/perseus,ariabuckles/perseus,iamchenxin/perseus,chirilo/perseus,learningequality/perseus,learningequality/perseus,abe4mvp/perseus,ariabuckles/perseus,abe4mvp/perseus,ariabuckles/perseus,ariabuckles/perseus,daukantas/perseus