Custom config: handle default export

[shrpne]

It should finally fix #806

Added interop default, because currently importing of custom config with export default is not working properly.

Also reorganized custom config logs a bit:

• logging "found custom config" only if it really loads
• otherwise showing error, instead of silently ignoring invalid custom config

[smorimoto]

Thanks!

[xAl3xFx]

In order for this to work with windows you need to add "file://" before process.cwd().