Provide a way to not use the source directory for relativeTo

[XrXr]

@shinnn In old versions of broccoli-clean-css relativeTo can be set to anything. However in newer versions it is always relative to the source directory.

Libraries that use the old behaviour are blocked from upgrading because of this (ember-cli/ember-cli-preprocess-registry#6)

Can we get some way to bypass this forced rebasing?

[rwjblue]

I believe that the behavior of relativeTo in broccoli-clean-css is correct even though it was a breaking change to ember-cli (which was absolutely fine to do in a major version bump). See my detailed explanation in ember-cli/ember-cli-preprocess-registry#6 (comment).

ember-cli is being updated to use the correct value for relativeTo in ember-cli/ember-cli#5463.

[XrXr]

Cool! There is no problem then.