Improve CMake setup

[lunacd]

Fixes #115

This PR makes changes to the CMake setup of this project, that should bring the following benefits:

1. It's easy now to package this project, should anyone be interested. The CML of this project now configures and installs all the CMake config and metadata that find_package expects. Consuming a packaged and installed version of this library is as easy as:

find_package(cpp-subprocess REQUIRED)
target_link_libraries(<target> PRIVATE cpp-subprocess::subprocess)

2. Even without packaging, this new setup is easier to consume for CMake. The cmake needed to consume this library is updated in README. The benefit of doing that over vendoring a copy of cpp-subprocess directly into somebody's project, is to easily consume upstream updates without having do the copying all over again and potentially ending up with a confusing git log with first-party and upstream changes mangled together.
3. CMake now reflects the version of this library.
4. The include path of this project is now consistently #include <cpp-subprocess/subprocess.hpp>, as documented in README. It can be slightly confusing if sometimes this project is called subprocess.hpp and sometimes it's cpp-subprocess/subprocess.hpp, at least within the official repo itself.

[arun11299]

@lunacd Awesome! Thanks for your contribution.