Remove extra modelines from included source files

[jayvdb]

The modeline on the output file should appear only once at the top of the file, and any subsequent modelines should be removed.

https://www.python.org/dev/peps/pep-0263/
https://wiki.python.org/moin/Vim

When templates include another template which has a modeline, multiple modelines may appear.

[chfw]

similar to your previous request to remove trailing newlines.

[chfw]

removing multiple mode lines should be a custom operation.

[jayvdb]

Similar to #38 , a plugin.