Specify config in an independent file

**Is your feature request related to a problem? Please describe.**
We are migrating our projects to a single monorepo and would love to have only one configuration set for all the stuff related to import sorting. Now, as ignoring files are only supported to be specified in the `pubspec.yaml` file we have to replicate that config per project instead of just once at the root of the monorepo.

**Describe the solution you'd like**
We would love to specify the config of `import_sorter` in a YAML file which could be passed using a relative path to the sort command.

**Describe alternatives you've considered**
I've tried to build a negative REGEX and pass it to the command (e.g. sort all imports here except those ending in `*.g.dart)`, but without success. If such a regex is possible, no config file would be needed as all the config options could be passed in one global command.