Add the facil.io framework

The facil.io framework is an evened framework written in C.

The framework is used to power the [iodine Ruby server](https://github.com/boazsegev/iodine) which focuses on serving real-time applications.

It's MIT licensed and free (as in freedom, as well as - I guess - beer).

Visit the [website for more information](http://facil.io).

Networking: Add libmicrohttpd

CLI: Add linenoise

Networking: Add Wangle

Serialization: Add upb

JSON: Update LIBUCL license info

Multimedia: Add libav

Graphics: Add TinySpline

Scripting: Add sol2

Add OpenXRay engine

spack was relicensed to dual Apache-2.0/MIT

add xmake

Multimedia: Add TagLib

Add xtensor to Math category

Added compile time regular expressions

A library presented at this year's CppCon https://www.youtube.com/watch?v=QM3W36COnE4

added Pixel Game Engine

Added meta, a runtime reflection library

Added meta: header-only, non-intrusive and macro-free runtime reflection system written in C++17. MIT license.

libhydrogen & YAS added

subj

Added cpplint to the list of static code analysis tools.