Prior to this change, only sass files starting with _ were ignored by
Zola's sass compiler. This made using sass frameworks incredibly
inconvenient, as Zola attempted to compile every single sass file in the
framework, and inevitably errored due to them not being standalone. For
example, to use the Bulma framework, you had to manually add an
underscore to the beginning of *every* sass file in it so Zola would
stop trying to compile them as standalone css files. Now you can change
the directory name to _bulma and have the same result.
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