Hyde update should reflect unknown tags in the YAML/JSON

[sean-parent]

The front matter in a Jekyll file may contain non-related tags to hyde. For example, the stlab libraries have a tag examples: that lists a set of example files to be referenced. Hyde update strips these tags. Another example is category: tags. It might be best if non-jekyl specific hyde tags themselves were name spaced so as not to conflict with other systems:

layout: function
title: at_pre_exit
owner: sean-parent
hyde:
    brief: Register the `pre_exit_handler` to be called on [`pre_exit()`](/libraries/pre_exit.hpp/f_pre_exit.html).
    tags:
      - function
    defined_in_file: pre_exit.hpp
    overloads:
      void at_pre_exit(stlab::pre_exit_handler):
        arguments:
          - description: __OPTIONAL__
            name: f
            type: stlab::pre_exit_handler
        description: Register the `pre_exit_handler` to be called on [`pre_exit()`](/libraries/pre_exit.hpp/f_pre_exit.html). The functions are called in the reverse order that they are registered.
        return: __OPTIONAL__
        signature_with_names: void at_pre_exit(stlab::pre_exit_handler f)
    namespace:
      - stlab
      - v1