Add Validate of config objects read from: config file, command line, env var, app override

[tohagan]

Need to design/implement a simple validation process based on the schema defined in defaults.config or perhaps another schema file. Needs to allow apps to use their own schema validation methods.

Ideas:

• Basic validation: checks that attributes in each config source (file, command line, env var) correspond to a named attribute in defaults.config file and reports invalid named attributes.
• Define a set of named validation functions that an app developer can augment with additional named validation functions.
• Define the named validation function that applies to each schema attribute.
• Allow additional validation function args to be passed to validation function for each schema attribute.