fix: only append .js to file.path when not already present

Currently a file's path gets appended '.js' each time that file is
processed by this plugin.  When karma is run in interactive mode with
debug logging turned on file paths will grow over time to look like:
`index.html.js.js.js.js`.

This doesn't seem to cause any errors in itself, but further downstream
processors may get confused as the file name keeps changing.

Closes #36