Allow running scripts via wave run

#### Is your feature request related to a problem? Please describe

On the most recent customer call, the customer wrongly assumed that Wave apps can be run via `python my_app.py` command because they saw it in our hello world tutorial, that presented a Wave script.

#### Describe the solution you'd like

Although this is a user error, it could be prevented by having a single way of running both scripts and apps - via `wave run`. IMO running scripts via `python script.py` is the most intuitive to python developers, but our audience is mostly not so tech-savy.

cc @mtanco @lo5 for thoughts

Start by reading the existing `wave run` command and the hello world tutorial's script execution example. Determine how `wave run` should distinguish and launch scripts versus apps, then verify that both forms run through the same command as described.

Written by the indexing model from the issue text.