host/OS aware sub configuration support

[christianparpart]

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include_configuration: "/home/username/.config/contour/config.d/${OSNAME}-${HOSTNAME}/*.conf"

The idea here is, that our contour.yml is the base config, but we can override things in here using a special include file, that is loaded after this file, and overrides items that were already loaded.

To make it more flexible, the user can use substitution variables (as shown above), e.g.:

• "${OSNAME}" -> "Windows", "Linux", "macOS", default uname based on what the syscall is returning us
• "${HOSTNAME}" -> so people can have hostname specific sub config overrides
• .... anything else that would make sense?