warn if undefined variable is used

[itajaja]

an undefined variable is silently copied through. Is there a way to fail or show warnings? Many times an undefined variable is a just a typo

[am80l]

We just found a variable that has been in our output for months due to this issue. postcss-simple-variables supports a silent : true/false option. We will be switching to that project since this is a critical feature.

[mjschlot]

I have completed the upgrade and submitted a pull request:
#37

[jonathantneal]

This is in 2.0.0. Happy New Year! https://github.com/jonathantneal/postcss-advanced-variables/blob/master/CHANGELOG.md#200-december-31-2017