Add properties supporting environment variable interpolation

[wwerner]

Adds a java.util.Properties extension that allows for interpolating values with data from the environment. Useful for adhering to https://12factor.net/config.

Value Syntax: ${<env var name>[:<default value>]}

Examples:

• a=${FOO}; FOO=bar -> a=bar
• a=${FOO}trag${BAZ}; FOO=un; BAZ=bar -> a=untragbar
• a=${FOO:qux}; FOO=bar; -> a=bar
• a=${FOO:qux}; FOO=<not net>; -> a=qux

Required for https://github.com/vlingo/vlingo-schemata/issues/112

[wwerner]

Ouch, sorry. I put < into the javadoc, should become green now.