Support .properties files

[johnwright]

Despite being rather anti-12-factor, .properties files are a useful fallback for testing, e.g. on a developer machine.

Explicit control over where to find the files is preferable.

If classpath properties files are allowed by default, resolution order should be: classpath /application.properties → environment variables → system properties. The idea behind this is that a file in the classpath could provide defaults that could be overridden.