Switch back from spec to source file, instead of creating a spec for the spec.

Currently, once you switch to/create a spec for a source file, running `spec-maker:open-or-create-spec` again creates a new spec for the spec.

This pull request modifies the `spec-maker:open-or-create-spec` command to detect that the current file is a spec file, and returns to or opens the source file the spec file tests.