Deprecate usage without root `$` (and provide explicit option to require)

[brettz9]

Given the potential for ambiguity (e.g., with a property named $), I think it is better to require the form $.aProperty instead of allowing the bare form aProperty.

[brettz9]

I think this would be more trouble than it is worth...I'll just document the need to escape literal $ (anywhere in the path) with the backtick, i.e., as `$.