Add script command to allow calling external helper scripts

HELP.md

@@ -129,6 +129,10 @@ Restore a `str` session from a saved state file (default `~/.str.json`).
 #### `runfile <path>`
 Execute the contents of the given file as a series of `str` commands.
 
+#### `script <path> [...<args>]`
+Execute an external script and replace the current subject with the returned output.
+The current subject is passed as the first argument, followed by the optional additional `args`.
+
 
 ### Basic String Operations