Get config parameter AllCops/Excludes from highest config file in path.

This is a solution for #288. The behavior was perceived as a bug by
@SirLenz0rlot and I agreed. We can discuss whether this change is
important and should be merged. It does introduce some added
complexity by handling `AllCops`/`Excludes` differently from all other
configuration parameters.

Look upwards towards root for a `.rubocop.yml` and take `Excludes` value
from highest file. This solves the problem of `.rubocop.yml` files in
directories that were supposed to be excluded shadowing the file that
excludes them.

Also fix inherited relative `Excludes` paths so they are relative to the
`.rubocop.yml` file that inherits them.