[Hunterize] Add support for installs, with a project config

[bkotzz]

This change enhances the CMake script to support installing the package so that other projects can use it.

• This adds support for a config file. This is the conventional way for a cmake project to support use by other projects via cmake find_package. See https://gitlab.kitware.com/cmake/community/wikis/doc/tutorials/How-to-create-a-ProjectConfig.cmake-file for more information.
• This adds support for an install target. You can then run make install, and it will install the static library and headers for other projects to consume, the generated config files, and the binaries for all of the apps in the project. The install locations are conventional (/usr/local/{include|bin|lib}) but can be overriden by the consuming package, or a package manager.

I've tested the basic ability to build against the include directory and link against the static library, and be able to use the library from another project.

This is also sufficient for use with package managers like https://docs.hunter.sh/en/latest/index.html.

PTAL @barbibulle