Keyword not found recommendations

For issue #888.
It would probably be possible to take the idea in my previous comment one step further and always pass candidates to `_get_close_matches` as a dict with keys being normalized names. Nowadays `_get_close_matches` creates such `norm_candidates` mapping internally. This would require some further changes too, but would probably remove more duplication. If you @guykisel are already bored with this (I wouldn't be surprised... =), I can take a look at that once we get this pr otherwise ready and merged.