* export tempDir
* add the initial implemetation of the `.play` virtual command
* re-arrange the cmd run loop to avoid some potential races when there are multiple commands
* allow playCmd to return an error early, that way we don't discard the error when the pkg import fails and cause an error about `undefined cmomand .play`
* pass the input file's fn to the exec method
* add default_commands object to store default commands. this is to work-around the the user settings overwriting the default ones
* rename the `run` command to `play` and replace their implementation with the `.play` implemented by margo/exec