Keyword not found recommendations

For issue #888.
Isn't `cutoff=0` somewhat too low? When looking at the updated tests, I was surprised with recommendations like this:

    No keyword with name 'Varargs not last' found. Did you mean one of these?
            BuiltIn.Variable Should Not Exist
            BuiltIn.Log Variables
            BuiltIn.Get Variables
            BuiltIn.Get Library Instance
            BuiltIn.Set Tags

I don't have time now to play with different cutoff values, and don't have any earlier experience from using difflib, but this is definitely needs some fine tuning. Great to see that there is a standard library method that seems to work well enough for our needs, though.