Add support for a directory source of config fragments

Allow multiple config fragments to be pulled in from a config directory, as opposed to individual config files being explicitly enumerated.

This logic could be enabled using a new --config-dir=/path/to/config CLI option taking a path to a configuration directory from which all contained *.conf fragments are loaded.

This would provide an alternative to multiple --config-file options when it is inconvenient to explicitly enumerate all the config files in advance, for example when an unknown number of config fragments are being generated by a deployment framework.