Allow script filename to omit the extension if it is .ff or .pe.

When processing a call in the native scripting environment, and no
script or built-in function is found to match, look for a script
ending in the extension .ff or .pe.  This allows scripts to be used as
function calls without needing to specify the extension in the source.
Merge pull request #1051 from blipvert/fixes/19

Allow script filename to omit the extension if it is .ff or .pe.