Allow run file specs to work with nested apps

[nathanKramer]

This allows VS Code projects with multiple app directories (e.g rails engines etc), or rails apps that are not at the root of the project, to work. It introduces the assumption that the rails root is called "app", and that this may not be the first entry in the path.

For example:

core/app/...
core/spec/...
microservice_A/app/...
microservice_A/spec/...

[rpellerin]

Would love to see this merged! It's very useful for Rails engines.

[nialbima]

I'm going to fork and publish this patch, because I use this shortcut constantly and need it.