The less plugin did its job, but when I was going to use it with a
library I experienced that it was lacking the ability to modify
less-variables. This made it hard to customize less libraries.

By letting the user provide a `options` in `gatsby-config.js`, the
plugin can overwrite variables defined in the less stylesheet and hence
makes it a breeze to customize libraries.

The user has two options to include vars: either a object defined
directly in the `gatsby-config.js` or define a file which exports an
object that will be used as the options.