Fallback values for variable expansion (":-" syntax)

[anuraaga]

Is your feature request related to a problem? Please describe.

Currently only ${IDENTIFIER} is supported for variable expansion and we can't set a value to an existing var if set or fixed value if not set.

Describe the solution you'd like

Parse ${IDENTIFIER:-VALUE} and use VALUE when IDENTIFIER variable not set

Describe alternatives you've considered

Additional context

I understand supporting every single type of variable expansion may be too much for this tool but does it seem reasonable to support just :-, which from what I've seen is the most commonly used?