* Pass `dirname` as extra metadata to preset constructor.
Sometimes a preset would like to know where it should resolve relative paths from (e.g. https://github.com/tleunen/babel-plugin-module-resolver) and this extra information makes that possible.
* Test for `dirname` passed into preset constructor
This adds a check for `dirname`’s existence and correctness to the
`resolve-addons-relative-to-file` test, and serves as a minimal example
of a path-aware preset.
