This pull request configures this repository to be run on Repl.it. It adds a `.replit` configuration file and a Repl.it badge to the `README`.
You can read more about running repos on Repl.it [here](https://docs.repl.it/repls/dot-replit), or view the Repl [here](https://repl.it/@shiv213/yaml-cpp).
* CMake: Prefix options with "YAML" and hide platform-specific options
When including yaml-cpp as a subproject, some option names can conflict with other projects.
(1) Make sure the yaml-cpp options are prefixed with YAML
(2) Hide platform-specific options when possible to avoid cluttering the cmake option list
* Update docs for change from BUILD_SHARED_LIBS to YAML_BUILD_SHARED_LIBS
