Consider replacing libmicrohttpd with a header-only library

[jslee02]

This idea was stemmed from #128 by @psigen (comment):

Perhaps now would be a good time to think a bit if a header-only implementation would be useful here, since the requirements are relatively simple (serving static file content on-demand).

https://github.com/yhirose/cpp-httplib

[psigen]

Here are some more candidates:

• https://github.com/ipkn/crow
• https://github.com/cpp-netlib/cpp-netlib