Accept dir and file after /-/

This changes adds the delimiter /-/ to the step reference short form which separates the repository URL from the (optional) path and filename where the step is defined within. If the path ends in .yml the last element is taken to be the filename. Additionally the path is assumed to start with "steps" (a breaking change).

In order to allow for flexibility, the long form retains the behavior in which an unspecified path points to the repository root (rather than /steps). The action-runner and scripts still have step.yml at the root so this change updates the hardcoded reference to the long form.

Fixes gitlab#509900 (closed)

Fixes gitlab#509901 (closed)

Fixes gitlab#509902 (closed)

Fixes #16 (closed)