Option to ignore unresolvable mixins

[gossi]

Given there are multiple configurations for postcss that apply for your project, it can happen that postcss-mixins runs multiple times but the second call has the configuration for the first iteration... then the build can fail, e.g. if the mixin is not resolvable. What would be nice is an option to ignore them, similarly to what postcss-advanced-variables does.

There is an option how to handle unresolved variables, see unresolved option.

My suggestion is to add this to postcss-mixins, too.

[ai]

Sure. Feel free to send PR.

I do not see a problem with this extra option, but to be honest, it look like a hack. We can add it as a quick fix, but it will be a good idea to think about refactoring to fix the initial problem of multiple plugins in the chain.