Import base *-variables files as fallback vars when loading packages/themes

Previous to this, theme authors needed to replicate the entire `ui-variables` or `syntax-variables` files in their themes. This pull makes it so they can just include the variables they want to override in their version of the `*-variables` file.

It didnt work like this because we would just pass a stack of import directories to the less compiler, and the first variables file won. If something used a variable not defined in that winning `*-variables` file, it'd die. 

This pull always imports the [fallback variables](https://github.com/atom/atom/tree/master/static/variables) on all package / theme files before importing anything else.

Fixes #2892; Requires atom/less-cache#3