lastgenre: Optionally replace unicode characters when performing lookups

It looks like there's a bug in lastgenre plugin that causes artists/albums with Unicode characters to be skipped:

    $ beet lastgenre 'albumartist:Sleater‐Kinney'
    genre for album Sleater‐Kinney - The Woods (None):
    $

My quick fix for this was to patch the plugin to add a new `asciify` option which, if `True`, runs unidecode() over the arguments before passing them onto the underlying pylast library:

    # post-patch
    $ beet lastgenre 'albumartist:Sleater‐Kinney'
    genre for album Sleater‐Kinney - The Woods (album): Rock
    $

I'm not sure if this the best solution. There could be cases where substituting characters returns an entirely different result than the one you expect.

A more thorough approach would be to try once with regular characters then again with `asciify`-ed characters if nothing is found (maybe prompt the user to confirm the new results are correct?). It shouldn't be too difficult to add this later if it's required.

This seems like the right place to do the replacement; good thinking. :+1: Can you please update the method name and docstring to reflect that it also cleans up characters in `args` (which now must be Unicode strings) to be a better match for the Last.fm database?