Improve handling of non-ASCII characters in import_wordpress script

The `import_wordpress` management command crashed on several UTF-8 characters I had in my Wordpress XML export file.

This code runs the relevant bits of XML through a try/catch that attempts to decode to ASCII, then asks the user to manually translate to ASCII if it can't.

Open to suggestions on an automated way to do this - I don't quite understand the mapping of UTF-8 onto ASCII, but doing it manually was sufficient for my purposes.